1. In a bowl, mix flour, 1/2 cup sugar, baking powder, baking soda, and salt.
2. With a pastry blender or your fingers, cut or rub butter into flour mixture until no pieces are larger than 1/4 inch.
3. In a small bowl, mix buttermilk and egg yolks; add to flour mixture. Stir with a fork until evenly moistened, then press into a ball.
4. On a lightly floured board, knead until dough forms a smooth ball, 5 to 10 turns. Pat dough into a 1-inch-thick oval. With a floured round cutter (2 1/4 to 2 3/4 in. wide), cut dough; place pieces about 2 inches apart on a baking sheet, about 12 by 17 inches. Press scraps together, knead briefly, and repeat to cut more rounds. Sprinkle cut dough evenly with sugar, using about 1 tablespoon total.
5. Bake in a 400° oven (or a 375° convection oven) until golden, 12 to 15 minutes. Transfer to racks.
6. In a bowl, mix fruit with sugar to taste. In another bowl, whip cream with a mixer on high speed until it holds soft peaks. Sweeten to taste with sugar.
7. Split warm or cool shortcakes in half. Set bottom halves on plates and spoon about half the fruit and juices onto cakes. Top each with a spoonful of cream. Set a cake top on each base and spoon remaining fruit and cream equally over each.