1. Put butter and sugar in bowl and beat with mixer until smooth, about 2 minutes.
2. Add the 2 whole eggs and the 2 yolks while mixer is running - do this slowly - and continue mixing for about 1 minute.
3. Add the lemon juice while mixer is running. The batter will look curdled. This is normal; it will change and get smooth as it cooks.
4. Put mixture in heavy saucepan over LOW heat, stirring until smooth; then turn the heat up to MEDIUM and cook, stirring constantly. Mixture will thicken, but it will take about 12 to 15 minutes; you'll know it's ready when it coats the back of a wooden spoon well. Mixture should never boil.
5. Remove the saucepan from the stove and stir in the lemon zest. Spoon mixture into a bowl and cover surface of lemon curd with plastic food wrap, pressing the plastic onto the surface of the curd - refrigerate.