Blue Velvet Cupcakes  2 1/2 c flour,  1 tbsp cocoa powder,  1 tsp baking soda,  1 tsp baking powder,  1 tsp salt,  1 1/2 c sugar,  1/2 c butter,  2 eggs,  1 c buttermilk,  2 oz blue food coloring,  1 tsp distilled white vinegar,  1 tsp vanilla extract