How to Make It
Prepare the Pastry: Place flour, sugar, cocoa, and salt in a food processor; pulse until combined, 3 to 4 times. Cut butter into 1-tablespoon pieces. Scatter butter pieces over flour mixture; pulse until mixture resembles coarse sand, 5 to 6 times. Drizzle 5 tablespoons of the coffee over flour mixture. Process until dough forms a shaggy mass, 10 to 20 seconds. If necessary, add remaining 1 tablespoon coffee to bring dough together.
Scrape dough out onto a lightly floured work surface. Knead until dough comes together, 3 to 4 times. Divide dough in two pieces; shape each piece into a 1-inch thick disk, wrap in plastic wrap, and chill 30 minutes.
Prepare the Filling: Combine brown sugar, half-and-half, butter, flour, and salt in a saucepan. Bring to a boil over medium, stirring constantly. Cook, stirring constantly, until mixture thickens, about 1 minute. Remove from heat; stir in bittersweet and unsweetened chocolate until smooth. Cool completely, about 20 minutes.
Preheat oven to 350˚F. Unwrap 1 piece of dough, and place on a lightly floured surface. Roll out dough to slightly thinner than ¼-inch. Cut into 8 (4- x 3-inch) rectangles, rerolling scraps once as necessary. Repeat with second batch of dough. Scoop a scant 1 tablespoon Filling into center of 8 rectangles; spread Filling over dough, leaving a ½-inch border. Prick remaining rectangles all over with a fork; place on top of Filling-covered dough, and crimp edges with tines of a fork.
Place pies on 2 parchment-lined baking sheets. Bake until pies are puffed and bottoms are slightly browned, 20 to 25 minutes. Cool on sheets 5 minutes; transfer pies to a wire rack, and cool completely, about 20 minutes.
Prepare the Glaze: Whisk together powdered sugar, vanilla, and 1 tablespoon of the water in a small bowl until smooth. Add additional water, 1 teaspoon at a time, until desired consistency is reached. Drizzle Glaze over cooled hand pies.