Pastry Cream (Crema Pasticcera)

Mix sugar and flour in small heavy saucepan. Whisk in milk until smooth. Cook over medium low heat, stirring constantly, until thick and smooth, about 5 minutes; remove from heat.

Gradually whisk a third of the milk mixture into egg yolks in small bowl. Whisk egg yolk mixture into remaining milk mixture in saucepan. Cook over very low heat, stirring constantly, until thick and smooth, about 3 minutes. Do not boil. Remove from heat.

Strain pastry cream through sieve into small bowl. Stir in vanilla and lemon zest. Cover surface of pastry cream with plastic wrap. Let cool to room temperature. Discard lemon zest.