For bottom layer, stir together oats, the 1/2 cup flour, the brown sugar, and baking soda. Stir in the 1/2 cup melted butter. Pat mixture into the bottom of an ungreased 11x7x1-1/2-inch baking pan. Bake in a 350 degree F oven for 10 minutes.
Meanwhile, for middle layer, stir together egg, granulated sugar, the 2/3 cup flour, the milk, the 1/4 cup melted butter, 1 ounce melted chocolate, the 1 teaspoon vanilla, and the baking powder until smooth. Fold in chopped walnuts. Spread batter over baked layer in pan. Bake about 25 minutes more or until a wooden toothpick inserted in center comes out clean. Set on a wire rack while preparing top layer.
For top layer, in a medium saucepan heat and stir 1 ounce chocolate and the 2 tablespoons butter until melted. Stir in the powdered sugar and the 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 walnut halves. Cool completely on wire rack. Cut into bars. Makes 32 brownies.
Prepare and bake brownies as directed. Frost and cool. Cover and store at room temperature for up to 1 day.