In a small bowl, whisk together the egg yolks and heavy whipping cream, and set aside.
In a large bowl, mix together the flour, cocoa powder, sugar, and salt. Add the cold butter. Using your hands, gently toss the flour mixture over the butter to coat.
Once the mixture is a crumbly texture, drizzle the egg and cream mixture over it and combine with a fork, until no dry flour is visible. It will likely be sticky at this point. But, don't worry, it will come together.
Turn the dough out onto a floured surface. You should coat the outside so that it no longer sticky. Be sure not too add too much excess flour. You will want to stop just when it is easy to handle without sticking to your fingers.
Divide the dough in half, and wrap each piece in plastic wrap, and form into a disk. Refrigerate for at least 45 minutes before using. If you are not planning on using it right away, wrap it in a double layer of plastic wrap and keep it in the refrigerator for up to 3 days. Alternatively, wrap it in two layers of plastic wrap, place in a freezer zip bag, and store in the freezer for up to 2 months.
Remove one dough disk from the refrigerator, and place it on a lightly floured work surface. Roll the dough into an approximately 13 inch diameter circle (about 1/8 inch thickness). This gives a 2 inch (2 cm) border for the sides of the tart. To check sizing, you can gently place your tart pan right on top of the circle to ensure that you have excess dough for the border.
Once you are satisfied with the size, starting at one end of the circle, gently place the rolling pin on top, and roll the dough around the pin like a roll of tape. You can use this transfer the dough to the tin without tearing it. Again, starting on one side of the tin, gently unroll the dough onto the tin.
Lightly press the tart dough into the tart pan, focusing on the corners, and working up the sides of the tin. Don't worry if it tears, you can easily patch it up with some excess dough.
Use the rolling pin to roll over the tin and remove any excess dough hanging over the edges. Place the tart tin and dough in the refrigerator for 25 minutes to chill.
Preheat the oven to 325 degrees F (165 degrees Celcius).
After 25 minutes, remove the tin from the refrigerator, and using a fork, make pricks in the dough covering the bottom of the tin. This will prevent it from bubbling up during baking, and you won't need to use pie weights!
Bake in a preheated oven for 25-30 minutes. Remove the tart crust from the oven, and allow to cool on a wire rack for 10-15 minutes.
Carefully remove the tart crust from the pan by placing your hand in the center of the pan, and allowing the ring to fall down your arm. You can remove the bottom of the tin if you wish. The tart should slide off easily. But, if it needs coaxing, gently run a sharp knife between the crust and the bottom of the pan to remove the pan's base. Place the tart back on a wire rack to finish cooling completely before filling.