In a medium saucepan, melt butter with water over medium heat. Bring to a boil, then stir in flour and salt. Cook until smooth and dough pulls away from sides, about 1-2 minutes.

Let the dough cool for 10 minutes, then add eggs one at a time, mixing until smooth and glossy.

Preheat oven to 350°F (175°C). Pipe small mounds of dough onto a lined baking sheet and bake for 25-30 minutes until puffed and golden.

In a saucepan over medium heat, whisk together milk, sugar, cornstarch, and vanilla. Cook until thickened, about 5-7 minutes.

Once cooled, slice pastries in half and fill with custard. Dust with powdered sugar if desired.

Serve immediately or store in an airtight container in the fridge for up to 3 days.