Whisk together sugar, eggs and egg yolks in a medium bowl until very well incorporated. In a medium saucepan over low heat, combine lemon juice and butter. Stirring often until butter is completely melted. Slowly whisk in egg & sugar mixture. Stir constantly. I use a wooden spoon for this... Continue stirring until it thickens enough to coat the back of your spoon. This takes about 5-10 minutes. 
Remove from heat and transfer the curd to a smaller heatproof container. This is when I fold in the zest. Then cover with plastic wrap. Please press the plastic wrap onto the surface of the curd - this will keep it from developing a plastic like film. 

Serve this with fresh baked scones or crepes or your favorite shortbread cookies.