1. Grease the baking pan with a little bit of butter, set aside. Make the filling: in a medium bowl, combine the brown sugar, the 5 tablespoons butter, and the cinnamon. Using a pastry cutter, cut the butter into the sugar, working it in until the mixture looks like wet sand. Put in the refrigerator to chill while you prepare the dough.
2. In a small bowl, whisk together the buttermilk and egg, set aside. In a medium bowl, whisk together the flour, sugar, baking powder, salt and baking soda. Sprinkle the butter over the four mixture and use to your fingers to work it in. Stop when the mixture looks like sand studded with little chunks. Add the egg mixture and stir with a fork just until the dough begins to hold together. It will look quite ragged and not fully blended, but stop there. You want to prevent the butter from melting into the dough-those little chunks will create a flakier texture once baked.
3. Preheat the oven to 350˚ and set a rack to the middle position. Dump the dough out onto a piece of parchment paper and knead just enough to bring it all together into a ball. Using a rolling pin and a bench scraper roll the dough onto a 9x15 inch rectangle with straight sides
4. Sprinkle the dough all over with the brown sugar mixture, leaving a 1-inch border across one of the longer edges. Top with the apples and gently press down. Working from the long edge opposite the border, roll the dough up tightle, jelly roll style, using the parchment as an aid. When you reach the border give the roll a squeeze and turn seam side down.
5. Cut the roll crosswise into 9 equal buns and arrange in the prepared pan. Bake until golden brown and bubbling, 30-35 minutes. Serve warm, right from the pan.