To produce light cakes, it is vital to measure out the ingredients accurately and stick to the recipe. If you add too much egg, butter, flour or baking powder, this will make the cake heavy and doughy.
It is air that makes cakes light, so beat the sugar and butter until the mixture is pale, soft and fluffy, and fold in the flour gradually, being careful not to knock out any air. And be sure to use an appropriate fat - if you use soft tub margarine instead of butter in a creamed recipe, the mixture will be too wet and the cake will be flat.
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