1. Make the biscuits: Preheat oven to 450 degrees. In a food processor, pulse flour, granulated sugar, baking powder, baking soda, and salt until combined. Add butter, and pulse until largest pieces are the size of peas. Add cream cheese, and pulse again until largest pieces are the size of peas. Add buttermilk, and pulse until dough comes together. Transfer to a bowl, and gently knead into a ball. Cover with plastic wrap, and let rest in the refrigerator at least 1 hour (or overnight).

2. Divide dough in half. Working with one portion at a time on a lightly floured work surface, pat into a disk, then roll into a 10-inch round. Transfer to baking sheets lined with parchment paper.

3. Brush biscuits with buttermilk. Generously sprinkle raw or sanding sugar over tops and sides. Brush off sugar from parchment. Bake until dough has risen slightly and is golden brown, 12 to 15 minutes (if bottoms brown too quickly, reduce temperature to 400 degrees). Leave biscuits on sheets. Using a 2 1/2-inch star cutter, cut out (but do not remove) 7 stars from one biscuit while it is still warm. Let cool.

4. Using a paring knife, carefully remove star shapes. Reserve for snacking or another use. (Biscuits can be made up to 8 hours ahead; store on wire racks, and cover loosely with plastic wrap.)

5. Make the filling: Combine blueberries, strawberries, and 2 tablespoons sugar, and let macerate until juicy, 10 minutes.

6. Just before serving, beat cream to medium peaks with remaining tablespoon sugar and the vanilla.

7. Assemble the cake: Place uncut biscuit on a large plate. Spoon half the berries and their juices on top of biscuit, then spread the whipped cream over berries. Top with all but about 1/2 cup berries. (Make sure surface is even; it will support the top biscuit and prevent it from breaking.) Carefully place cutout biscuit on top. Fill star cutouts with reserved berries. Serve immediately, or refrigerate, uncovered, up to 3 hours.