For the Chocolate Sable: This sable crust recipe will provide you with more than enough dough than you need for the Hot Chocolate Tart. You can either cut the recipe in half or make the full amount and freeze the remaining dough for up to two months.
In a mixer with the paddle attachment on low, mix together the flour, cocoa powder, sugar, and butter, blending until sandy.
In a separate bowl whisk together the yolks, cream, and vanilla extract. Add to the dry ingredients and mix until dough just comes together. Make into disks, wrap, and chill for two hours.
Preheat oven to 375 degrees F.
Roll out in small batches, as it gets soft fast. With a large round cutter, cut out disks and keep them chilled while you roll and cut the rest of the dough.
Line 4-inch tart rings with the disks and chill. Line the rings with plastic wrap (that's right!) and fill with rice, then gather the plastic wrap up to contain the rice. Bake for 14 minutes, then remove the rice packets and bake 2 more minutes to crisp. Let cool slightly while you make the filling.
To make the filling: Melt the chocolate and butter over simmering water.
Meanwhile, whisk together the eggs and yolks. Whisk in the powdered sugar to the egg mixture until most of the lumps are gone. Whisk in the melted chocolate mixture, and lastly, the flour.
Pour filling into tart shells and bake for 8 minutes. The center should still shimmy and be loose and melted.
Serve immediately sprinkled with powdered sugar and cocoa powder and ice cream or whipped cream.