Combine 3/4 cup sugar and butter in a bowl; beat at medium speed of a mixer for 3 minutes.
Add milk and liqueur; beat well. Lightly spoon flour into a dry measuring cup; level with a knife.
Combine flour, 1/4 cup cocoa, baking powder, and salt in a bowl; gradually add to sugar mixture, beating until well-blended. Spoon batter into an 8-inch square baking pan coated with cooking spray.
Combine 1/4 cup cocoa, 2/3 cup sugar, and coffee granules in a small bowl; sprinkle over batter.
Pour boiling water over batter (do not stir).
Bake at 350 for 30 minutes or until cake springs back when touched lightly in the center (cake will not test clean when a wooden pick is inserted in center).