Combine Brownie Mix and yogurt, stir until it has a glossy finish. Stir in walnuts.
Spread batter into a foil lined 8 in square baking pan. Bake at 350 for 28 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on wire rack.
Place butter, vanilla, and milk in a medium bowl; beat with a mixer at medium speed until smooth and creamy. Add powdered sugar, beating at low speed 1 minute or until preading consistency. Gradually add melted chocolate, beating until smooth. Spread frosting evenly over brownies.
Remove brownies from pan by lifting foil. Remove foil; cut into 16 squares.