Begin by preheating your oven to 350°F (175°C).
In a saucepan, bring water, butter, and salt to a boil. Add flour all at once, stirring vigorously until the mixture forms a smooth ball.
Remove from heat and allow to cool slightly. Then, incorporate eggs one at a time, ensuring each is well mixed before adding the next.
Divide and spread the dough on a baking sheet into two equal rectangles.
Bake for about 30 minutes, or until golden brown, and then let cool completely.
Heat milk with sugar and vanilla in a saucepan, stirring until the sugar is fully dissolved.
In a separate bowl, blend cornstarch, flour, and egg yolks. Gradually whisk in some of the hot milk mixture to temper, then return everything to the saucepan.
Cook over low heat, stirring constantly, until the cream thickens.
Remove from heat, stir in butter until smooth. Let the mixture cool, then refrigerate until chilled.
Spread the chilled cream over one layer of the cooled dough, then carefully place the second layer on top. For a finishing touch, dust with powdered sugar.
