1 1/2 cups white sugar
1/4 cup butter
2 1/4 cups quick-cooking mix
2 1/4 cups all-purpose flour
2 teaspoons baking powder
4 teaspoons baking soda
1/2 teaspoon salt
1 cup butter or margarine, chilled
2 teaspoons vanilla extract
3/4 cup confectioners' sugar
1/2 cup unsalted butter at room temperature
1/4 teaspoon vanilla extract
Preheat oven to 350 degrees F (175 degrees C). Grease a 10x15 inch baking dish. In a medium bowl, stir together the soda/baking powder mixture and baking soda. Stir in the flour, 1 tablespoon at a time, until just sticking. Mix in baking powder, baking soda and salt.
Separate the butter or margarine and sugar into 1/2 cup and 1/4 cup portions. Stir well until merely stiff peaks form. Transfer mixture to the prepared half-sheet pan.
Pour 3/4 cup of the butter or margarine mixture over the entire pan. Roll, pinch together, the flour, baking powder, baking soda, salt, and 1 cup of the rolled butter or margarine mixture over the batter in the pan.
Bake 15 to 20 minutes in the preheated oven, or until a knife inserted into the middle comes out clean. Remove from the oven and place on a wire rack to cool completely before cutting into squares.
To prepare the topping: Beat the butter or margarine until smooth and light. Stir in vanilla extract and confectioners' sugar until smooth and spreadable. Frost with confectioners' sugar, then sprinkle over frosted chocolate to brighten.