Whisk the flours, sugar, baking powder, soda and salt together in a bowl.  Drop in the butter and then rub it into the flour mixture with your fingertips until the butter is in little pea sized bits.  Whisk together the egg yolk and buttermilk.  Stir into the dry ingredients until the dough pulls together.  Tip it out onto a lightly floured surface and knead a couple of times, then pat the dough out into a 10 by 6 inch rectangle.  Spread lightly with softened butter.   Mix together the cinnamon and the sugar.  Sprinkle about 2/3 of this mixture over top of the butter, pressing it lightly to help it adhere.  Roll up from the long side as if you were rolling up for a cinnamon roll.  Press the roll into a rectangle which is 10 inches long by 3 inches wide and 1 1/2 inches tall.  Cut in thirds and then cut each third in half diagonally into a triangle.  Transfer to the baking sheet.  Lightly butter the top of each triangle and sprinkle with the remaining cinnamon sugar.