Add flour and cocoa powder to the bowl and pulse it twice to combine it.
Add the chilled and cubed butter and blitz it for about 20 seconds. The butter will now be turned into marble size lumps.
With the food processor on low, drizzle cold water in until it begins to clump.
Shape the dough into a disk with cold hands and wrap it in plastic. Refrigerate it for 30 minutes (or 15 minutes in the freezer).
In the meantime preheat the oven to 355F/180C and grease the sides and bottom of a 8-inch round pie dish (with removable bottom).
Dust the countertop with a bit of cocoa powder and roll out the dough to a 3 mm/ ⅛ inch thickness.
To transfer the dough to the pie dish lift up the dough with a wide spatula and slide the greased bottom of the pie dish under it. Now fold the 'overhang' towards the center of the pie bottom so all the dough is supported by the bottom and it can't break off. Transport it to the pie dish and once it is in, fold back the extra dough (so now it is draped over the sides of the pie dish).
Try not to press down the dough (too much) and only use a fork to make a few holes in the bottom to prevent air bubbles to pop up while baking.
Use a rolling pin to cut off the extra dough that is hanging over the edge by rolling it over the top. See picture in the text above to see how.
To blind bake the pie crust, add baking beans (or other type of baking weights) to the pie dish, but not before you have lined the dough with parchment paper so the beans don't stick to the pie crust.
Bake for 15 minutes with the baking beans and after removing the baking beans and parchment paper bake for another 10 minutes. Once reading, take the pie crust out of the oven, but leave the oven on.
Let the pie crust cool off. In the meantime continue with the brownie batter.