CAKES BULGE BETWEEN LAYERS AFTER ICED
I decided it would be a good idea to rework this file I wrote for Diane Shavkin once not long ago and post it so you could have a well-formatted file. What prompted this was that a lady had written Diane wanting to know what she could do about icing bulging out in the middle. She had frozen her cake. I felt that my answer was incomplete. I have also added 'How to Ice a Cake" too, for your benefit.

The Reasons:
   Cakes develope bulges when they have been iced BEFORE they have a chance to settle. Gravity says outer edges ARE going to settle down some....even when you DO have the layers perfectly level. If there is a bulge, the outer edges of the cake won't stand up in mid air. It is going to settle down some. So give time for this to happen before you ice the cakes.  Even push tiny cake scraps in between if there are large gaps.
  I don't usually have gaps because I bake at 325 deg. and so my cakes don't puff up so much in the center. Then to get rid of the hump, I lay a kitchen towel on top just as soon as I remove it from the oven and gently press the layers flat. (This needs to be done within the first minute upon removing from oven). Oh - the towel thing won't work if you over bake the cake...it will just spring back up, besides it is going to be dry and tasteless...so DON'T over bake. Test the cake by pressing your finger gently in the center. If it <barely> springs back up - its done...Take it out.
  At times you may still have a slight hump on the outer edge that can cause a bulge. Then I shave that part off with a serated-edged knife.

Procedure to follow:
Bake layers, cool well.
Fill or put icing between...PLENTY of icing too. (medium consistency is fine).

• Let the cakes set several hours or overnight before icing them.
• Oh yes - always bottoms UP on those layers will help with this problem too. There are always going to be bulge problems if you insist on putting bottoms together. There is no 'breathing' room with this procedure. Besides, it is a lot easier to ice the flat bottom of a cake than one you have had to shave flat.

• Cut the end off a parchment cone (or use decorator bag with no tip on the coupler).
• Fill bag with medium-thick icing.
• Pipe ring of medium consistency icing around edge of bottom layer in order to prevent filling from seeping through the icing.
• Allow icing to crust 30 minutes or so.
• Spread filling generously inside ring using a small bent spatula.
• Position the top layer of the cake, bottom-side-up, over the filling.
• Let the cake set for awhile to settle layers together before icing it.

NOTES:
It is not advisable to split layers and fill the wedding cake layers.
They may slip while you are delivering the cake.
Just put filling between the layers.

Some people or books also call “spreading icing between the layers” - “filling the cake.”

Bulges (showing between the layers), which develop after the cake is iced are caused when the cake develops a hump while baking. If a hump does form, just shave it off so the cake appears nice and level on top before you turn it out of the pan.

•      The Magic Cake Strips help greatly in reducing humps. If you still have humps (after pressing the cake with a towel as above), shave them off with a serrated-edged knife.

• Calculated icing recipe is based on the use of  1 cup shortening to 2 pounds confectioners’ sugar. With flavors and liquids included. This will be enough to fill, ice the cake and add the decorations. It won’t quite take one batch of icing per recipe or cake mix when doing larger cakes.
• Have icing medium consistency for spreading. ***If icing is too stiff, it will pull off cake crumbs into your icing. You can add water or milk to thin it down.

• DIAGRAM, METHOD 1 below, shows the usual way to ice a cake (with an 11-inch cake spatula). It will take some practice. I will also give you several other alternatives.
• Use a straight firm-bladed spatula. Spreading the icing on is done in 2 steps.
• First, spread a thinner coating of icing on (same consistency!), pressing hard, to seal in all the crumbs. Remember, This coating should be thin, not thickly spread. The crumb coating seals the cake.
• Second, go right back over the cake and add a second coating immediately, of the amount needed to coat evenly with no cake showing through. Scrape off and remove excess icing from sides. Follow this method by the steps below.
• TIP: If you leave the cake set awhile between crumb coating and final icing, you may find crusted icing pieces showing in the final coating. CRUMB COAT FIRST
• I cover my BARE cakes (with layers together) using a covered cake rack. But you can drape a shower curtain over them, or place them in a garbage bag etc. Let cakes set overnight. Then I crumb-coat and final-ice.

METHOD 1 - WITH A SPATULA
1. SIDES FIRST: Pile enough icing on top of the cake, near the outer edge, to cover. Pull some icing down over the side and holding spatula as shown in Diagram 1, pulling icing around to coat thinly.

Whenever you run out of icing (and need to pull some more down), swivel your spatula in the other direction and gradually pull away (to move the knife away without pulling crumbs off). Finally, pull some more icing over the edge and repeat until covered thickly.

SPECIAL TIP: If you hold the spatula so only the edge is against the cake, rather than flat with the whole blade touching, you will find that most air holes will smooth out as you go.

2. SMOOTH OUT THE ICING: Hold the spatula straight up and down and moving clear around the sides of the cake. Don’t press to hard on the icing now, just smooth it gently. Remove any extra icing from the sides as you go.

3. ICE THE TOP: Keep blade of spatula against outer edge of cake and pull icing in towards the center into a pile. Remove excess. Then sweep the spatula, three times across the top only if possible, lightly down the center. Press lightly so you don’t pull off too much icing.

• Cake should finally be coated with just enough icing so cake barely does not show through the icing.
• An 11-inch spatula spreads icing nicely on cakes. The blade is firm enough to keep it from bending and long enough to ice the sides.
• If you can’t get icing smooth enough with the spatula, smooth more using either a smooth paper towel, facial tissue or dress maker’s interfacing *: Place against cake, rub gently.
• Additional smoothing can be achieved by laying a paper towel on the cake (after 5 minutes or so to allow icing to crust just a little), and rub lightly in a circular motion until smooth. We like SCOTT or VIVA brands of paper towels for their texture. Wavy patterned ones cause problems trying to match the “waves.” Almost smooth paper towels are available for smoothing wedding cakes.

Diagram 1, Step 1
You will get the perfect amount of icing on the cake every time.
The CAKE ICER TIP is the very best tool for a novice decorator. It still takes some effort but you will be pleased with the outcome.
It is especially wonderful for icing the sides of those character cakes that require smooth sides!

NOTE: Diagram shows going around the cake backwards from the way you should go. This was done for picture taking purpose only. ALWAYS draw the bag of icing towards you, not away.

Diagram 2, Step 2
Use a large decorator bag, 16-inch or larger, with medium consistency icing. The bag opening must be cut to fit the tip. Be careful not to cut too much off, as the bag will stretch over time. No coupler will fit the cake icer tip. Do the sides first, then the top, as shown above. Smooth out the lines made by the tip using a spatula. Press lightly while smoothing. For character (shaped) cakes, usually no smoothing is needed at all We LOVE it!