You should not beat egg yolks at high speed to start with

Step by step recipe

1	To avoid this, you should start beating gently, without switching on your mixer, just stir it round. By doing this, you will break egg yolks and start to mix them with sugar.
2	Once this premix is done, you can switch on the mixer and beat as normal. Don't forget that you should not leave egg yolks in sugar to wait any length of time.