Combine topping ingredients together in a bowl and set aside. Preheat oven to 350 degrees. In small bowl, blend dry ingredients. Mix butter and sugar together with vanilla. Add eggs and flour mixture. Stir in sour cream and 1 1/2 cups of the chocolate chips. Spoon into prepared muffin tins, filling 1/2 to 2/3 full. Sprinkle with the topping mix. Bake 20 to 25 minutes. Makes about 18 muffins.