In a bowl, whisk together the flour and cocoa powder until combined.
In the bowl of a stand mixer fitted with the paddle attachment (or with a hand mixer), cream together the butter, sugar and salt until smooth. Scrape down the sides.
Add in the egg and mix until incorporated, then scrape down the sides again.
Add the flour and cocoa powder and mix on low until just incorporated. Wrap the dough in plastic wrap and flatten into a 1-inch thick disc. Place in the fridge for two hours.
Remove the dough from the fridge and allow it to sit at room temperature for about 15 minutes. Flour your work surface and rolling pin and roll out the dough until it’s ⅛" thick, turning a quarter turn every few rolls. This dough is very forgiving, so if it cracks, you can patch it later.
Cut 12 circles out of the dough using a 3 ¼” - 3 ½” cookie cutter (it should be slightly larger than your tartlet pan). After the first round of cuts, mash the dough scraps back together, roll out to ⅛” thick again, and cut rounds. Repeat again, if needed.
Transfer the pieces of dough to the tartlet pans and carefully press the dough into the bottom and sides of the tartlet pans. Push into the dough so it rises almost to the top of the pan opening and you work your way around the pan. This is the same process if using a muffin tin. If using individual tartlets, place them on a rimmed cookie sheet. Freeze the tartlets for 15 minutes.
Preheat the oven to 325℉/163℃ and cut twelve, 4” pieces of parchment paper.
Remove from the freezer and dock all over the bottoms with a fork. Place a piece of parchment paper (it helps if you crumble it up first) into the shell, and fill completely with pie weights (or dried beans/rice, just don’t eat it afterwards), gently pressing the weights into the bottom and sides to evenly distribute the weight.
Bake for 10 minutes. Carefully remove the weights and parchment, and continue to bake for another 10-15 minutes until lightly golden. When done, the middle of the tart should not look moist or raw. Remove the pans to a wire rack to cool completely, then remove the crust from the shells before assembling. They should slip right out.