1/4 cup/55gms sugar
1 tablespoon/8gms all purpose flour
1/2 teaspoon finely grated lemon zest
1/2 teaspoon/ 2.5ml vanilla extract
1 large egg yolk
3/4 cup/175ml whole milk
Mix together the sugar, flour, lemon zest and vanilla extract in a medium heavy-bottomed saucepan. To this add the egg yolk and half the milk. Whisk until smooth.
Now place the saucepan over low heat and cook, stirring constantly to prevent the mixture from curdling.
Add the remaining milk a little at a time, still stirring constantly. After about 12 minutes the mixture will be thick, free of lumps and beginning to bubble. (If you have a few lumps, don’t worry. You can push the cream through a fine-mesh strainer.)
Transfer the pastry cream to a bowl and cool to room temperature. Cover with plastic film and refrigerate at least 4 hours or overnight, until thoroughly chilled.