Preheat your oven to 350 degrees.
In a medium mixing bowl, whisk together the dry ingredients (flour, powdered sugar, cocoa powder, salt).
Chop the cold butter into 1-2 centimeter cubes and add it to the dry ingredients. Use your fingers, forks, or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse sand. Any leftover butter chunks shouldn't be larger than a pea.
Add the egg and heavy cream and mix together until all of the dry ingredients have been coated. *I usually use my hands at the end of mixing to make sure everything is well incorporated
Work the dough into a ball and place it on to a clean, well-floured surface. Use a rolling pin to roll the dough into a circle large enough to cover your tart pan.
Carefully lift the dough and place it in your tart pan. Press the dough down, trim the edges off (no dough should be hanging over the side) and use excess dough to fix any holes.
Use a fork to evenly prick the dough on the bottom of the pan (8-10 times) to avoid bubbles when baking.
Bake the crust for 15-20 minutes or until the dough is no longer shiny and it appears cooked.
Remove it from the oven and set it aside to cool.