Stir together chocolate morsels and 1 tablespoon cake mix; set aside.
Beat mashed banana, 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 1 cup toffee bits evenly over top of batter; sprinkle Streusel Topping evenly over batter.
Bake at 350° for 30 minutes or until a wooden pick inserted in center of cake comes out clean. Cool on a wire rack.