1. In a food processor or bowl, whirl or stir flour, cocoa powder, 1/2 cup powdered sugar, and the brown sugar and salt until well combined. Cut 1 cup butter into 1-inch slices; add to flour mixture and whirl or rub in with your fingers until mixture is blended and crumbly.
2. Scatter dough evenly in a 10- by 15-inch rimmed baking pan. With your palm, pat dough to cover pan bottom. Bake in a 300° oven until slightly firm and dry to the touch, 30 to 35 minutes. Let cool completely on a rack.
3. In a bowl, beat melted chocolate, remaining 2 tablespoons butter, remaining 2 cups powdered sugar, and 3 tablespoons hot water until smooth. Spread evenly over shortbread.
4. Cut into 1- by 2-inch bars; sprinkle with crushed candy.
Note: Nutritional analysis is per bar.