Preheat oven to 350 degrees. Line a baking sheet with parchment paper.
In a small bowl, combine warm water and yeast. Mix with whisk to dissolve yeast; set aside for five minutes while yeast blooms.
Sift flour, granulated sugar, baking powder, baking soda and salt into a medium bowl. Add shortening. Use a pastry cutter or fork to mix until coarse and crumbly.
Add the yeast-water and buttermilk, and mix with a wooden spoon, until just combined. (Do not overmix).
On a floured surface, pat dough into a rectangle, approximately 9 x 5 inches. Dough will be sticky.
To achieve a flaky, layered effect, it's important to give the dough a series of tri-folds: fold into thirds like a letter, folding the left third over the center third first, and then the right third on top.
Sprinkle more flour and roll out to a 9 x 5 rectangle again, repeating the tri-fold step twice more (for a total of three tri-folds), ending with a rectangle 3/4 to 1 inch thick.
Cut into six equal rectangles (or rounds) and place on the baking sheet. Brush with egg wash and sprinkle on coarse sugar.
Bake on center rack for 18-20 minutes, or until golden brown. Cool for ten minutes.
Serve with your favorite preserves.