Heat oven to 325°F. Grease 13x9-inch pan. In large bowl, beat 3/4 cup powdered sugar and 3/4 cup margarine until light and fluffy. Stir in melted cooled chocolate. Lightly spoon flour into measuring cup; level off. Add flour, baking powder, salt, 1 tablespoon milk and 1/4 teaspoon mint extract to margarine mixture; mix well. Dough will be stiff. Press dough into greased pan. Bake at 325°F. for 15 to 25 minutes or until center is set. Cool completely.
In large bowl, blend all topping ingredients except chocolate sprinkles; beat until smooth. Spread over base. Sprinkle with chocolate sprinkles. Cut into bars. Store in refrigerator.