Chocolate mousse tart

Crust ingredients:
1/2 stick unsalted butter, at room temperature
1/4 cup (60gr) unsifted powdered sugar
1 egg yolk
pinch of salt
1/2 cup +2 tbsp all-purpose unbleached all-purpose flour
2 tbsp unsweetened cocoa powder

Mousse ingredients:
1 box Dr Oetker dark chocolate mousse
1 cup cold milk

Prepare the crust:
In a mixer, whip together the butter and sugar together until light and fluffy. Add the egg yolk and pinch of salt and mix until incorporated. Add the flour and cocoa powder and mix briefly. Dump the whole mixture onto a lightly floured board and gather the dough into a smooth ball. Do not work the dough while in the mixer or it will toughen it up. Flatten the dough into a disk, wrap it in plastic wrap and refrigerate for an hour.
Preheat oven to 350F and position a rack in the center.
When the dough is nice and cold, roll it out on a lightly floured board or in between two sheets of plastic. Cut out four 6-inch-rounds of the dough and fit them into four individual four inch tart pans. If the dough tears while you roll or/and transfer into the rings, just patch it with your fingertips. Line the dough with pieces of parchment paper, fill with pie weights or dry beans and bake for 10 minutes. Remove the weights and parchment paper. Cool crust completely.

Prepare chocolate mousse:
Prepare mousse according to directions on box.

Assemble tarts:
Fill crust with mousse and refrigerate for at least four hours or overnight.

Recipe adapted from Tartelette