Preheat the oven to 350 degrees F. Lightly roll out 1 piece of dough into a 12-inch circle on a floured surface. Cut out circles using a 3-inch-round cookie cutter. Repeat with the other piece of dough. Fit the dough circles into the holes of a 24-cup mini-muffin pan, pressing into the bottom and side of each cup. Lightly brush the dough with the beaten egg and sprinkle with cinnamon sugar. Bake until browned, 16 to 18 minutes; let cool in the pan.
Combine the chocolate, peanut butter and confectioners' sugar in a medium bowl. Heat the heavy cream in a small saucepan over low heat until steaming; pour over the chocolate mixture and add the vanilla. Let stand until the chocolate softens, 3 to 5 minutes, then whisk until smooth and glossy. (If the chocolate is not completely melted, microwave in 20-second intervals, whisking, until smooth.)
Spoon the chocolate mixture into the pastry cups and decorate with nonpareils. Let stand at room temperature until the chocolate sets, about 1 hour, then unmold.