1 cup milk chocolate toffee bits or 4 Heath candy bars (1.4 ounces each) , chopped
In a large bowl, combine the first four ingredients; cut in butter until crumbly. Set aside 1/2 cup for topping. Stir baking soda into remaining flour mixture. Combine the buttermilk, egg and vanilla; stir into flour mixture just until moistened.
Pour into a greased 13x9-in. baking pan. Add the toffee bits to the reserved crumb topping; sprinkle over batter. Cut through batter with a knife to swirl the topping.
Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack.