Blend flour, brown sugar, cornstarch, 1 tablespoon espresso powder and salt in a food processor. Add butter and vanilla. Using the pulse cycle-or pressing on/off button on your food processor, process until mixture resembles coarse meal. Add nuts; blend until finely chopped. Transfer dough to floured work surface. Knead just until dough comes together.
Divide dough in half. Press each half into 9-inch-diameter tart pan with removable bottom.
Bake at 350 degrees F (175 degrees C) until deep golden brown, about 25 minutes. Transfer to a rack and let cool for 2 minutes then remove sides of pan. Cut each shortbread round into 24 wedges. Cool completely.
Mix 2 tablespoons hot water and remaining 1 teaspoon espresso powder in small saucepan. Add chocolate. Stir over medium-low heat until chocolate is smooth. Remove from heat. Cool slightly. Drizzle chocolate mixture over cookies. Let stand until chocolate sets.
