Preheat the oven to 450F; set rack in middle level. Prepare biscuit dough. For a large shortcake, pat the dough into a 9-inch disk on a parchment-lined cookie sheet. For individual shortcakes, pat dough into a 6-by-12- inch rectangle and cut into eight 3-inch biscuits with a sharp, floured knife. Transfer to a parchment-lined cookie sheet. Paint tops with buttermilk. Sprinkle with sugar. Bake for 10 to 15 minutes, until well risen and golden. Check the center of the large shortcake with a toothpick to make sure it is baked through: If the pick emerges with dough still clinging to it, lower temperature to 350F and bake another 5 minutes. Slide the large shortcake onto a platter immediately after it is baked. Use a sharp, serrated knife to slice through the middle, making two layers. Slide the edge of a cookie sheet between the two layers and lift the top layer off. Butter the bottom layer and pour all but 1/2 cup of the sweetened fruit on. Slide the top back on and pour the remaining berries over the top. Split the small shortcakes in the same way and place the bottoms on individual dessert plates. Butter them and top with about 1/3 cup of the fruit mixture. Replace tops and pour a tablespoon or so of the remaining berries over the top. Serve immediately. Pass the cream in a bowl for the guests to help themselves.
BUTTERMILK BISCUIT DOUGH: COMBINE THE FLOURS, salt and baking powder in a mixing bowl and stir well to mix. Rub in the butter by hand or with a pastry blender until the mixture is mealy. Stir in 3/4 cup of the buttermilk with a fork and continue stirring gently until the dough begins to hold together. (If the dough is dry, add more buttermilk, 1 tablespoon at a time.) Sprinkle the work surface generously with flour and scrape the dough onto it. Fold the dough over on itself two or three times. Use the dough immediately for shortcakes, following the instructions above. Use this recipe to make a large shortcake or eight small ones. This same dough is used as the topping in cobbler recipes.