In the bowl of a stand mixer fitted with a paddle, beat butter on medium speed until creamy. You can also use a hand mixer.
Add the sugar, all at once and beat on medium speed until creamy and combined.
Scrape down the sides of the bowl and add the egg and egg yolk, one at a time, and mix well after each one.
Add the flour, cocoa, and salt and mix on medium speed until the dough comes together.
Put it in a plastic film and refrigerate for at least 1 hour or overnight.
Preheat the oven to 350F (180C).
Roll the dough between two pieces of parchment paper until it's about 0.2 inches (5 mm) thick.
Press the dough onto the bottom and edges of the tart pan. Cut the excess dough. Prick it all over with a fork.
Refrigerate for 15-30 minutes.
Cover the dough (bottom and edges) with parchment paper. Fill the center with pie weights, rice or beans.
Bake for 15-20 minutes until firm. Let it cool on a rack.
If you have leftover dough, you can make some stars with a cookie cutter for decoration.