Heat milk. Whisk sugar, yolks, cornstarch, and salt. Temper with milk and cook until thick. Stir in butter and vanilla. Chill.

Boil water, butter, and salt. Add flour and stir until a ball forms. Cool briefly.

Add eggs one at a time until smooth and glossy.

Pipe dough onto lined baking sheet and bake until puffed and golden.

Cool shells, then pipe chilled vanilla cream inside.