Combine 1/2 cup flour, oats, and next 4 ingredients; stir well. Press flour mixture into an ungreased 9" square pan. Bake at 350° for 12 minutes.
Combine 1/3 cup butter and chocolate in a small saucepan. Cook over medium-low heat until melted, stirring frequently. Let cool.
Beat eggs at medium speed of an electric mixer until thick and pale. Add 1 cup sugar and Kahlúa, beating until blended. Stir in chocolate mixture, remaining 3/4 cup flour, and walnuts; spread mixture evenly over crust.
Bake at 350° for 25 minutes. Cool in pan on a wire rack. Spread Coffee Frosting over brownies. Cover and let stand until frosting hardens. Cut into 16 squares. Garnish with chocolate coffee beans, if desired.