Roll out the dough and fit into a 10-inch fluted tart pan with a removable bottom, or smaller individual tartlet pans. Trim any excess dough from around the edges. Prick the bottom of the shell all over with the tines of a fork. Line the tart shell(s) with a piece of buttered aluminum foil, butter side down, and fill with dried beans. Bake for 10 minutes. Cool slightly.
For Mascarpone Filling: In a large bowl, combine mascarpone, sugar, eggs, vanilla, and cornstarch. Whisk together until smooth and creamy. Spoon mascarpone mixture into the baked tart shell and bake for an additional 25 minutes.
To Assemble: Allow the tart to cool. Decorate with blackberries. Refrigerate until ready to