In a large bowl, combine the flour, powdered sugar, cocoa powder, and salt.
Add the diced butter and blend in using two forks, your fingers, or a dough blender until combined with a crumbly texture and no pieces are larger than the size of a pea.
Slowly add the cold water just until the dough comes together.
Form the dough into a flattened disc, wrap in plastic, and refrigerate for at least 30 minutes or up to 2 days.
Preheat oven to 350˚F (180˚C). Grease 12 tartlet pans with butter.
On a lightly floured surface, roll the refrigerated dough into an even, thin circle about 1/8 inch (3 millimeters) thick.
Cut out circles of dough using a cutter slightly larger than the width of the tartlet pans.
Press each circle into the prepared pan. Bring together the scraps of dough and roll out again, then cut if needed to make more circles.
Cover each circle of dough in the pan with parchment and add pie weights or uncooked rice. Bake in the preheated oven for 15 minutes.
Carefully remove the pie weights and parchment from the pan and continue to cook the crust for another 5 minutes. Remove from the oven and set aside.