Chocolate Chocolate Muffins

sugar 3 Tbsp butter, melted and cooled 3 Tbsp buttermilk 1/2 tsp vanilla 3 Tbsp well-beaten egg 1/3 cup + 3 Tbsp flour 1 Tbsp + 1 tsp cocoa powder 1/2 tsp baking powder 1/4 tsp baking soda 1/8 tsp salt 1/2 cup chocolate chips