1Preheat oven to 350°F. In the bowl of a food processor, combine flour, brown sugar, cocoa, cornstarch, espresso and salt. Add butter and vanilla and pulse until mixture resembles coarse meal. Add nuts; blend until finely chopped. Transfer dough to floured work surface. Knead just until dough comes together.
2Divide dough in half and press each into an 8" or 9” round cake pan, or tart pan with removable bottom. If you like, press around the edge with the tines of a fork. Bake for 20-25 minutes, until set. Cool on a wire rack, then cut each shortbread round into 12 wedges.
3If you want to drizzle your shortbread with chocolate, put the chocolate chips into a zip-lock baggie and seal. Place in a bowl of very warm water and let sit until melted. Knead the bag a bit to make sure there are no chunks left. When smooth, snip a tiny corner off and drizzle chocolate over cookies. Let stand until chocolate sets.