How to Make It
Grate the frozen butter using a box grater. Place the grated butter in the freezer while preparing the rest of the dough.
In a large bowl, whisk together the egg, egg yolks, custard powder, and half of the granulated sugar. Whisk well, until the mixture lightens in color.
On a lightly-floured surface, roll the dough into a rectangle that’s ¼ to 1/6 inch thick. Be sure to sprinkle with flour to prevent the rolling pin from sticking (if the dough sticks to the surface or the rolling pin, it will mess up the layers). Place the rectangle on a parchment-paper lined baking sheet. Use a fork to prick all over. Let the dough rest in the refrigerator for 45 minutes. Preheat the oven to 350°F.