Is your sponge sinking? If your cake is heavy, there must have been a problem somewhere along the way. Making a light sponge is all about incorporating air into the mixture and maintaining that aeration. If the air is knocked out, or it isn’t evenly distributed, you’re left with a dense cake. By identifying where the problem has occurred, it’s much easier to correct it next time.
It could be in the eggs, if they are not whisked enough with the sugar, the mixture will not be light. If they are over-whisked, the structure will collapse. Try whisking a small amount of egg and sugar until it becomes thick and when the beaters are lifted, the mixture holds a ribbon that slowly pours back onto itself. Observing this will help you know when the eggs are ready.
If the cake is dense, it may also be down to overmixing when you add the flour. Stirring too much will knock all the air out of the eggs. Instead, gently fold the mixture together with a spatula. Try adding the flour in two goes, very gently. Taking your time will prevent the mixture from becoming dense. Overmixing or leaving the mixture to stand for too long can also affect the structure.
The mixture should be popped into a prepared tin as soon as it has been mixed and placed in the oven straight away. Leaving the mixture for just a few minutes can affect the structure of the finished cake. The more you do something, the better you get at it. Spend about fifteen minutes preparing a small sponge batter, just focusing on whisking the eggs and folding the flour in gently.
Bake it in a simple form and observe the crumb structure after cooling. Next time you try it, whisk the eggs for just a bit longer or fold the mixture a little more gently and see what difference it makes. If you make a dense cake, don’t automatically think it is the recipe; look back at how you mixed it. Was the folding a bit fast? Were the eggs whisked long enough? By practicing each stage separately and tweaking as you go, you will eventually find you are making light, evenly baked cakes that are perfect every time.