In a pot, beat together the egg yolks and 3/4 cup sugar until thoroughly blended and the mixture turns frothy. Add the cinnamon stick and grated lemon rind. Pour in the milk and add cornstarch. Slowly heat the mixture, stirring constantly. Heat just until mixture thickens. Then, remove pot from heat immediately.
Tip: As soon as the mixture thickens and you feel resistance while stirring, remove the pot from the heat or the mixture may curdle or separate! If that happens, the texture of the finished crema catalana will be grainy instead of smooth and creamy as it should be.