Preheat oven to 350 degree F. Grease a 9x9x2-inch baking pan and set aside. In a medium saucepan melt butter and chocolate over low heat. Stir in sugar, eggs, and mint extract. Using a wooden spoon, beat lightly by hand just until combined. Stir in flour. Spread batter in prepared pan.
Bake for 20 minutes. Cool on a wire rack. Meanwhile for frosting, in a medium mixing bowl beat the 1/4 cup butter or margarine until fluffy. Gradually add the 1 cup sifted powdered sugar. Beat in creme de menthe. Gradually beat in about 1/2 cup sifted powdered sugar until the frosting is easy to spread. Spread atop brownies. In a small, heavy saucepan melt semisweet chocolate over low heat. Drizzle atop brownies. Let chocolate set, then cut into bars. Makes 16 bars.