Prepare the Choux Pastry:

• Preheat oven to 375°F (190°C) and grease two 9-inch round cake pans.
• In a saucepan, bring water and butter to a boil. Remove from heat and quickly stir in flour and salt until a smooth dough forms.
• Return to low heat and stir constantly for 1-2 minutes. Remove from heat and let cool slightly.
• Beat in eggs one at a time, mixing well after each addition, until smooth and glossy.
• Divide the dough evenly between the two cake pans, spreading it thinly to the edges.
• Bake for 25-30 minutes, until golden and puffed. Let cool completely.

Prepare the Cream Filling:

• Heat 1 1/2 cups of milk over medium heat until warm.
• In a separate bowl, whisk together sugar, cornstarch, egg yolks, and remaining 1/2 cup milk until smooth.
• Slowly pour the warm milk into the egg mixture, whisking constantly. Return to the saucepan and cook over medium heat until thickened.
• Remove from heat and stir in vanilla extract. Let cool slightly, then beat in softened butter until smooth.

Assemble the Cake:

• Place one choux pastry layer on a serving plate. Spread the custard filling evenly over the top.
• Place the second pastry layer on top and dust generously with powdered sugar.
• Refrigerate for at least 2 hours before slicing and serving.