Dissolve the yeast in the warm water and set aside.
Place the buttermilk in a small saucepan and heat over low heat for 4 or 5 minutes just until lukewarm, no more than 110 degrees. Remove from the heat and mix with the dissolved yeast, stirring to combine well. Set aside.
Sift into a large bowl 4 cups of the flour, the baking soda, baking powder, salt and sugar. Cut the butter into small chunks and add to the bowl. Using two knives or a pastry cutter, cut the mixture until it resembles coarse cornmeal.
Add the buttermilk-yeast mixture to the flour mixture, sprinkling over a little of the liquid at a time while stirring with a fork to moisten. Turn the dough out onto a floured cloth or board and form into a ball. Sprinkle with a little more of the remaining 1/2 cup of flour if the dough is sticky.
Knead lightly for about 2 minutes, sprinkling with a little more flour if necessary.
Roll out the dough into a 9-inch circle. Using a 2-inch cookie or biscuit cutter, cut into rounds. Roll the scraps and cut again, if needed.