Curdling occurs when the butter and sugar have not been creamed sufficiently - until light and fluffy - to form a strong emulsion to absorb the eggs. Also if cold eggs are added to the mixture too quickly, separation occurs. A curdled mixture holds far less air, so the cake will be flat. Eggs should be at room temperature and added gradually. Adding a teaspoon of flour with each addition of egg can also help to prevent curdling.
If the mixture starts to curdle, dip the base of the bowl into warm water briefly, and whisk to restore the light consistency.
** Asian Recipes