Preheat oven to 400°F (200°C). Cut pastry into circles and place on a parchment-lined baking sheet. Prick with a fork. Bake for 15–18 minutes until puffed and golden. Let cool.

Whisk milk, sugar, cornstarch, and salt in a saucepan. Cook over medium heat, stirring, until thick and bubbly.

In a bowl, whisk egg yolks. Slowly add ½ cup of the hot custard while whisking, then pour it all back into the pan.

Cook 5 more minutes, stirring until smooth. Remove from heat, stir in vanilla and butter. Chill with plastic wrap pressed on top.

Cut the cooled pastry circles in half. Spoon or pipe custard onto the bottoms, then add the tops.

Dust with powdered sugar and enjoy.