Whisk together the flour, baking powder, baking soda, salt and the remaining 2 teaspoons sugar. Add the yeast mixture, buttermilk and butter to the dry ingredients and stir with your hands to just combine, being careful not to overwork the dough. Cover and let rest in a warm area to rise, 30 to 40 minutes.
Preheat the oven to 425 degrees F. Butter a baking sheet.
Place the dough on a floured surface and knead until smooth. Roll out to 3/4 inch thick, and cut out biscuits with a biscuit cutter. (We use a 2 1/2-inch cutter.) Place the biscuits on the buttered baking sheet nestled together, and brush the tops with melted butter. Allow to rest and rise for 15 minutes.