In a stand mixer fitted with the paddle attachment, cream the butter, sugar and salt on medium speed until smooth.
Add the egg, then scrape down the sides of the bowl. Add in the flour and cocoa powder and mix on low until just combined.
Wrap the dough in plastic wrap and flatten into a 1-inch thick disc. Place in the fridge to firm up for 2 hours minimum. When ready, allow the dough to come to temperature for 15 minutes before rolling it out.
Lightly flour your work surface and rolling pin and roll the dough out, turning a quarter turn every few rolls, until the dough is about ⅛” thick and is 2 inches wider than your tart pan. Throw a bit more flour anywhere your dough may be sticking.
Transfer the dough to the tart pan and carefully press the dough into the bottom and sides of the tart pan, then trim off the excess so it’s flush with the top of the tart pan. Use the scraps to patch any weak spots or holes in your dough. Place the tart in the freezer for 15 minutes.
Preheat the oven to 325℉/163℃ while the tart freezes.
Remove the dough from the freezer and dock the bottom of the tart with a fork multiple times. Line the shell with parchment paper and fill completely with pie weights (or dried beans/rice).
Bake for 10-15 minutes. Carefully remove the weights and parchment, and continue to bake for another 10-15 minutes. When done, the middle of the tart should not look moist or raw. Remove the pan to a wire rack to cool completely.
Once the tart shell is cool, melt the dark chocolate in the microwave in 10 second increments, stirring in between each round until completely smooth and melted.
Using a pastry brush, spread the melted chocolate into a thin, even layer on the bottom and halfway up the sides of the tart. Only use the amount of chocolate you need to do a thin even layer, anything leftover should be discarded (into your mouth, preferably). Place the tart in the freezer for about 5 minutes to firm up the chocolate.