Crust: Using a large sieve or a whisk to sift the flour, powdered sugar, almond flour, cocoa powder and salt into a large bowl.
Add the cold butter to the sifted dry ingredients. Use a pastry blender to cut the butter into the flour mixture.
To the dry mixture add the beaten egg. Knead the ingredients together by hand for a few minutes or just until a soft dough is formed. It will still be a little sticky.
Transfer the dough to a piece of plastic wrap and form it into a disc. Wrap tightly. Place the dough in the fridge to chill for 10-15 mins.
Roll the Dough: Place the chilled dough between 2 pieces of parchment paper and use your rolling pin to roll the dough to 1/4-inch thickness.
Use a 3-inch or 4-inch perforated tart ring to cut out the base of the tarts. Place the rings on a parchment paper lined sheet pan or silicone mat. Lightly press the base of the tart into the bottom of the rings.
Re-roll any dough scraps. Cut strips of dough that are slightly wider than the edge of of the tart ring to allow for a slightly overhang. (If the dough gets sticky, pop it in the fridge for 5 minutes to chill before continuing.)
Lightly spray the sides of the tart ring with nonstick cooking spray. Wrap the dough strips around the inside of the tart rings pressing to adhere the dough to the ring and the bottom crust. Use a knife to trim the end of the strip to fit, but leave the excess over the top of the tart shell. Place into the freezer for 30 minutes.
Bake: Preheat the oven to 325°F. Use a sharp knife to trim the (now frozen) top of the dough so that it is flush with the edge of the tart ring. Use a fork to dock the base of the tart shell.
Bake for 12-15 minutes. The shell will be slightly lighter in color and will appear dry. Cool on the pan for 5 minutes. Remove the tart ring from the outside of the tart and carefully move to a wire rack to cool completely.
Chocolate Mousse: Place chopped chocolate in a large heat proof bowl.
In a small saucepan, heat 1 cup heavy cream until steaming and bubbles form around the edge of the pan, don't boil.
Pour the cream over the chocolate and let stand for 5 minutes, then stir until it's completely melted. Remove about 2/3 cup of chocolate to a separate bowl for the tops.
Use a hand mixer on medium-high speed to whip the remaining 2 cups heavy cream with the powdered sugar and vanilla extract until soft peaks form. Add the melted chocolate beating on medium-high speed until stiff peaks form.
Divide the chocolate mousse evenly between the cooled tart shells, then spread or drizzle, the reserved 2/3 cup chocolate ganache on top, garnish as desired. (If you chill the tarts to set the mousse first, warm the reserved ganache in the microwave just enough to make it spreadable. You can also add additional heavy cream to thin.)
Chill chocolate tarts for 4 hours until serving.