These shells are used with our recipe for Marbleized Chocolate Truffle Tartlets.
Preheat oven to 300 degrees. Pulse flour, sugar, cocoa, and salt in a food processor to combine. Pulse in butter and vanilla until mixture comes together. Form into a ball.
Roll out dough to 1/8 inch thick; cut out twelve 4-inch rounds with a cookie cutter, and fit into twelve 2 1/4-inch brioche molds. Place on a rimmed baking sheet; bake, pressing down crust halfway through baking, until firm, 25 to 30 minutes. Let cool completely; remove shells from pans.