In a small bowl, combine the flour and sugar; cut in butter until mixture resembles coarse crumbs. Stir in nuts; set aside.
In a small saucepan, melt chocolate over low heat. Stir until smooth; cool. In a small bowl, combine the flour, sugar, baking soda and salt. Add the sour cream, butter and egg; beat until blended. Remove 1 cup of batter; stir in chocolate.
Spread the remaining batter into a greased 8-in. square baking dish; spoon chocolate batter over the top. Cut through batters with a knife to swirl. Sprinkle with reserved nut topping. Bake at 350° for 35-40 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack.