Preheat oven to 350 degrees F. For bottom layer, stir together oats, 12 cup flour, brown sugar, and baking soda. Stir in 12 cup melted butter. Pat mixture into the bottom of an ungreased 11x7x1 1/2-inch baking pan. Bake for 10 minutes.
Meanwhile, for middle layer, stir together egg, granulated sugar, 23 cup flour, milk, 1/4 cup melted butter, 1 ounce melted chocolate, 1 teaspoon vanilla, and baking powder until smooth. Stir in chopped walnuts. Spread batter evenly over baked layer in pan. Bake 25 minutes more. Place on a wire rack while preparing top layer.
For top layer, in a medium saucepan heat and stir 1 ounce chocolate and 2 tablespoons butter until melted. Stir in powdered sugar and 1/2 teaspoon vanilla. Stir in enough hot water (1 to 2 tablespoons) to make a mixture that is almost pourable. Spread over brownies. If desired, garnish with chopped bittersweet chocolate. Cool in pan on wire rack. Cut into bars. Makes 16 to 24 bars.