Stir together chocolate morsels and 1 tablespoon cake mix; set aside.
Beat sour cream, milk, eggs, and remaining cake mix at low speed with an electric mixer 30 seconds or until blended. Increase speed to medium, and beat 3 more minutes.
Spoon half of batter into a lightly greased 13- x 9-inch pan; sprinkle morsel mixture evenly over batter. Spread remaining batter on top. Sprinkle Streusel Topping evenly over batter.
Bake at 350° for 25 minutes or until a wooden pick inserted in center of cake comes out clean. Cool on a wire rack.