Combine water, yeast and sugar. Stir to dissolve yeast and let stand until bubbly, about five minutes. Fit the food processor with a steel blade. Measure the flour, butter and salt into the work bowl. Process about 15 seconds, or until mixed. Add the yeast mixture to the flour and process until blended, about 10 seconds. With the processor running, very slowly drizzle enough buttermilk through the feed tube to form a soft ball that cleans the sides of the bowl. Process until the ball turns around the bowl about 25 times. Turn off the processor and let the dough stand for 2-3 minutes. Turn on the processor and gradually drizzle in enough remaining buttermilk to make a soft, satiny dough. Process until dough turns around the bowl about 15 times.
Turn the dough onto a lightly floured surface. Shape into a ball and place in a lightly greased bowl. Cover with plastic wrap and let stand in a warm place until doubled in size, about 1 hour. Punch down the dough. Shape into a loaf and fit into a 8 1/2 x 4 1/2 x 2 1/2 inch loaf pan. Cover loosely with plastic wrap and let stand in a warm place until doubled, about 45 minutes.
Heat oven to 375°. Bake the loaf until golden, about 25-30 minutes. Remove immediately from pan and cool on wire rack. Makes one loaf.