• Preheat oven to 400˚F. Line two baking sheets with parchment.

To make the pâte à choux:

• In a medium saucepan, add the water and butter over medium heat. Once melted, whisk in the flour, reduce heat to low, and cook, continuing to stir, until the dough lifts from the sides and bottom of the pan. Remove from heat and allow to cool.
• Beat the eggs into the dough, one at a time, until well-combined, followed by the baking powder.
• Divide the dough into two equal pieces. Place each piece on the prepared baking sheet and press into equal rectangles about 1/4 inch thick.
• Bake in the preheated oven until puffed and golden, 15 to 20 minutes. Do not open the oven door until it is nearly done. Remove and allow to cool to room temperature.

To make the vanilla cream filling:

• In a large saucepan, place the milk over medium heat. Once starting to steam, whisk in the flour, potato starch, and vanilla sugar (if using vanilla extract, wait to add until after removing from heat) until smooth with no lumps. Once bubbling, reduce heat to low and continue to whisk until the mixture has thickened to the consistency of pudding. Remove from heat and allow to cool to room temperature.
• In a large bowl, beat together the butter and sugar until light and fluffy. Slowly mix in the cooled milk pudding until smooth.
• Evenly spread the cream custard over on layer of the cooled pâte à choux, leaving an 1 inch border around the edges. Gently top with the other layer of dough. Refrigerate until chilled, then sprinkle with powdered sugar and cut into squares to serve.