This is the number one question I get asked in class and 90% of the time it is simply that the ingredients are not at the same temperature especially the eggs.
So what has actually happened and why does this make the batter curdle?
The butter and sugar when creamed reach a fairly warm temperature as the friction of the beating warms the butter allowing it to expand and dissolve the grains of sugar incorporating air and making a lovely light cake. The eggs should be added gradually (I usually add them one at a time and beat each in well before adding another) if they are at the same temperature as the room they should blend in easily. If the eggs are cold (never use them straight from the fridge) they simply re-set the butter in the cold liquid of the egg making a porridge like mess. At this point most people add some flour to “soak up” the liquid. This is huge mistake because as soon as you do this you form gluten strands which toughen your cake as you continue beating resulting in a tough heavy cake.
So how do I do this correctly?
1: Use a trusted recipe
2: Use butter at room temperature
3: Take the eggs out of the fridge at least 3 hours before you need them
4: Beat the eggs in gradually
5: Fold in the dry ingredients by hand OR fold in very slowly in the machine.