Note: This dough requires a one-hour chill time before being baked.
When ready to bake heat the oven to 350°
Whisk the flour, cocoa powder, powdered sugar, and salt in a large mixing bowl.
In a small bowl mix the egg yolk, and one tablespoon of the cream together.
Cut the butter into small dice and add to the flour mixture and toss to coat the butter with the flour.
Add the beaten egg yolk mixture to the bowl and using a pastry blender or two knives cut in the butter and yolk into the flour until a soft dough starts to form. At this point, the dough will look a bit crumbly.
Add the remaining cream and mix that in with a fork. At this point, the dough will still be crumbly. With your hands start pressing the dough together to form a cohesive dough
Turn the dough onto a rolling mat and lightly knead to bring the dough together. Form into a rectangle shape or a round shape depending on the type of pan, and wrap the dough in plastic wrap and refrigerate for an hour.
To roll out the dough: Remove the dough from the fridge and let it sit on the counter for a few minutes to soften slightly for easy rolling. Lightly sprinkle cocoa powder on a rolling mat, or use flour, and then sprinkle the top of the dough with a little cocoa powder or flour.Roll the dough for a round tart pan into a 12-inch (30cm) circle for a 9-inch or 10-inch pan. For a 14 x 4-inch rectangle tart pan roll the dough into a 16 x 8-inch rectangle. Next, gently roll the dough up around the rolling pin and then unroll the dough over the tart pan. Lift the excess dough from the sides to fill the bottom of the pan. Work your fingers around the pan to press the dough in the bottom and the side.
Using a fork, prick holes in the bottom of the crust.
Take the rolling pin and roll it over the top of the tart pan to remove the excess dough. If you have excess cocoa powder or flour brush it away with a pastry brush. Cover pan with plastic wrap and place in the freezer until firm, about 20 minutes. Freezing the dough makes it less likely it will shrink during baking.
Note: This dough is fairly soft and if you want feel free to press it into the tart pan. I prefer the rolling pin method that makes the dough nice and smooth.