Transfer the mixture to a saucepan and heat over medium heat, stirring constantly.

Once it reaches a pudding-like consistency, cook for 1 to 2 minutes, until no raw starchy taste remains.

Remove from the heat and whisk in the butter until fully melted and incorporated.

Transfer to a clean bowl and cover with plastic wrap pressed directly against the surface to prevent a skin from forming. Refrigerate until completely cold.

In a large bowl, combine the flour, salt, and sugar.

Add the butter cubes to the flour. Work the mixture with your hands until all the butter is incorporated into the flour and some large chunks remain visible.

Add a couple of tablespoons of water and mix until absorbed. Repeat until the dough comes together and is no longer crumbly. Do not knead, press and squeeze the mixture gently to bring it together. You may need slightly more or less water, so add it gradually until the dough forms.

Place the dough on a floured surface and roll it into a rectangle. Fold the top and bottom edges toward the center, leaving a small gap between them. Fold in half again to form a book shape (called a book-fold). Wrap in plastic wrap and refrigerate until firm.

Preheat the oven to 400F (205C). Prepare a baking sheet and two pieces of parchment paper.

Divide the chilled dough into two equal portions. Roll out one portion on a floured surface, or directly on floured parchment paper, into an 11 x 11 inch (28 x 28 cm) square. The dough will shrink during baking, so it needs to be rolled slightly larger than the final size.

Bake for 12 to 15 minutes, or until golden brown. Allow to cool for 10 minutes, then use a pizza cutter or sharp knife to trim it into a 9 x 9 inch (23 x 23 cm) square.

Repeat with the second portion of dough so you have two squares.

Whip the heavy cream with the powdered sugar to stiff peaks. Using the same mixer, whip the chilled pastry cream until smooth.

Fold the whipped cream into the pastry cream with a rubber spatula until fully combined.

To assemble, place one layer of pastry on a plate or serving board. Spread all of the pastry cream evenly on top using an offset spatula. Place the second pastry square on top and gently press from the center toward the corners. If some cream oozes out, do not worry. Run an offset spatula along the sides to push the cream back in and fill any empty spots, especially around the corners.

Tightly cover the top and sides with plastic wrap and refrigerate for 6 hours or overnight.

Remove from the fridge and cut into 9 squares using a serrated knife. The cream will still be soft, so cut carefully.

Dust with powdered sugar, if desired, and serve. This dessert is not meant to be eaten while held in your hand. No matter how stable the cream is, once, the cream will ooze out. So enjoy it on a plate, it will be a mess, but a delicious one.