Patience is key here. The key to smooth, creamy curd is to cook over medium heat and whisk constantly. This is not the time to walk away. Bring the mixture almost to the boil – it will thicken as it cools. If you don’t stir enough, or if you have the heat too high, you may end up with specks of scrambled egg in your curd. I recommend using a whisk rather than a spoon to stir the curd as it makes it easier to get into the edges of the pan.