How to level a cake
This technique from The Great New Zealand Baking Book's Jaz Nixon will help ensure you get even layers when you are splitting a cake to fill it.
Levelling a cake is the process of slicing the natural dome off the top so it is flat and easy to stack and ice. It also gives the top of your cake a smooth, sharp edge. For best results, always let the cake cool before levelling it. Once levelled, turn the cake over (base side up) before icing, as this naturally has a more level surface. Also, since the base has not been cut, fewer crumbs will get into your icing.
There are two ways to level a cake. Jaz recommends using a cake leveller if possible.
Using a cake leveller
These are available from specialty cake decorating stores and online. As well as being more accurate, using a cake leveller means you can ensure that each cake in a stack is exactly the same height. Place the cake on a flat surface, dome side up. Stand the cake leveller beside the cake and adjust the cutting wire so it is the same height as the base of the dome (where the side of the cake and dome meet). Adjust the cutting wire by moving the ends of the wire up or down the cake-leveller frame.
Keeping the frame of the cake leveller upright and on the flat surface on either side of the cake, use a gentle sawing motion to pull the wire through the cake. If the crust is hard, you might need to make a small cut at the beginning, using a sharp serrated knife, to help the wire to enter the cake.
Using a knife
Place the cake on a flat surface or turntable, dome side up. Using a sharp, serrated knife, gently saw off the dome, keeping the knife as level as possible as you cut. This method does not require any specialty equipment but it can be difficult to achieve an even, flat surface.
You may like to wrap a thick ribbon or length of baking paper around the cake (secured with a toothpick or pin) to use as a level guide for cutting evenly.