- Preheat oven to 350 F. For the bottom layer: In a small bowl, stir together oats, the 1/2 cup flour, the brown sugar, and baking soda. Stir in the 1/2 cup melted butter. Pat the mixture into the bottom of an ungreased 1171 1/2-inch baking pan. Bake for 10 minutes.
- Meanwhile, for the middle layer: In a medium bowl, beat egg with a fork; stir in granulated sugar, the 2/3 cup flour, the milk, the 1/4 cup melted butter, the 1 ounce melted chocolate, the 1 teaspoon vanilla, and the baking powder until smooth. Fold in chopped walnuts. Spread batter evenly over baked layer in pan. Bake about 25 minutes more or until a toothpick inserted in center comes out clean. Place on a wire rack while preparing top layer.
- For the top layer: In a medium saucepan, combine 1 ounce chocolate and the 2 tablespoons butter; heat and stir over low heat 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 spreading consistency. Spread over brownies. Garnish with walnut halves. Cool in pan on a wire rack. Cut into bars.