Combine 1 cup of butter and 1 cup of water in a saucepan over medium heat. Bring to a boil, then remove from heat and stir in 1 cup of all-purpose flour until a dough forms. Allow to cool for about 10 minutes, then incorporate 4 room-temperature eggs one at a time.

Preheat your oven to 375°F (190°C). Spread half of the choux pastry dough onto a lined baking sheet and bake for 30-35 minutes until puffed and golden brown.

Prepare the custard by whisking together 2 cups of milk, 1 cup of sugar, 1/4 cup of cornstarch, and 1 tablespoon of vanilla extract. Cook over medium heat until thickened, about 10-15 minutes. Let cool completely.

Assemble the cake by placing one baked pastry layer on a plate and spreading the cooled custard filling evenly. Place the second pastry layer on top.

Cover with plastic wrap and refrigerate for at least 2 hours, ideally overnight, before serving. Cut into squares to serve.