Prepare Your Ingredients: Measure out all your ingredients accurately. Keep the cold butter in the fridge until ready to use.
Mix Dry Ingredients: In a large mixing bowl, whisk together the powdered sugar, pastry flour, almond flour, and salt until well combined.
Incorporate Butter: Add the cold chunks of unsalted butter into the dry mixture and work it into the flour mixture until it resembles coarse crumbs.
Add Egg and Form Dough: Create a well in the center of your mixture, add the egg, and gently mix until just combined; shape it into a disc.
Chill Dough: Wrap your dough disc in plastic wrap and refrigerate for at least one hour or overnight.
Roll Out & Bake: After chilling, roll out your dough on a lightly floured surface to about 3mm thick and transfer it into tart pans.