In a mixing bowl, combine brown sugar and butter. Beat in eggs, one at a time. Add coffee and vanilla; mix well. Combine the flour, cocoa, baking powder and salt; add to sugar mixture and mix well. Stir in chocolate. Spread into a greased 13 x 9 x 2 inch baking pan. Bake at 350 for 25-30 minutes or until a toothpick inserted near center comes out clean. Cool on a wire rack.
For frosting, combine the butter, sour cream and vanilla; mix well. Gradually stir in sugar until frosting is smooth and reaches desired consistency. Frost brownies. Sprinkle with grated chocolate.