In a medium saucepan over medium heat, combine water, butter, and salt. Stir until boiling. Gradually add flour while stirring until a smooth ball forms, about 2-3 minutes. Remove from heat and let cool slightly before mixing in beaten eggs.

Preheat oven to 400°F (200°C) and line baking sheets with parchment paper. Divide pastry into two portions, smoothing each onto the sheets. Bake for 25-30 minutes until golden brown, then cool on wire racks.

In a saucepan, heat milk over medium heat until steaming. Whisk sugar, cornstarch, and egg yolks in a bowl until smooth. Gradually whisk in hot milk, then return to saucepan. Cook while stirring until thickened, about 5-7 minutes, then stir in vanilla extract.

Place one choux layer on a platter. Spread custard on top, then place the second layer over it, pressing gently. Chill in the refrigerator for at least 1-2 hours.

Dust the cake with powdered sugar before serving. Cut into squares and serve chilled.