For the bottom layer, in a medium bowl, stir together the oats, 1/2 cup flour, brown sugar, baking soda and cinnamon. Add the melted butter, and stir to combine. Press mixture into the bottom of a greased 9x13 pan. Bake for 8 minutes, or until mixture is bubbly and slightly browned.
While bottom layer is baking, prepare the brownie layer. In a large bowl, com the egg and granulated sugar, whisking to combine. With a wooden spoon, stir in the 2/3 cup flour. Add the milk, 1/4 cup butter, 1 ounce melted chocolate, 1 teaspoon vanilla, salt and baking powder, and stir until fully incorporated. Fold in the walnuts. Spread mixture over baked oat crust. Bake for 18-20 minutes or until a wooden pick inserted near the center comes out clean. Remove from oven and place on a wire rack.
For the frosting, place the remaining 1 ounce chocolate and the 2 tablespoons butter in a microwave-safe bowl. Heat on high for 1 minute, stirring after 3 seconds, until the mixture is fully melted. Stir well to combine. Stir in the 1/2 teaspoon vanilla. Gradually stir in the powdered sugar, mixing well, until the frosting is smooth and spreadable. (Add an additional 1-2 tablespoons warm milk if necessary if the mixture is too thick.) Spread frosting on top of brownie layer in pan. Cool completely before cutting into 16 bars.