Preheat your oven to 400°F (200°C). In a medium saucepan, bring water, butter, and salt to a boil. Stir in flour until dough forms and pulls away from sides, then let cool for 5 minutes.

Add eggs one at a time, mixing until glossy. Divide the dough between two greased 9-inch pans, spreading evenly.

Bake for 25-30 minutes until golden and dry to the touch, then cool completely.

Warm milk over medium heat without boiling. In a bowl, whisk together sugar, cornstarch, and egg yolks until smooth. Gradually mix in warm milk, then return to saucepan and cook until thickened.

Let the custard cool to room temperature, then beat in softened butter until creamy.

Spread custard filling over one pastry layer and top with second layer. Chill in the fridge for at least 2 hours.

Dust with powdered sugar and serve chilled with coffee or tea.