Preheat the oven to 425°F, positioning a rack in the center. Line a baking sheet (13x18-inch) with parchment paper.

In a large mixing bowl, whisk together the flour, cornstarch, baking powder, baking soda, sugar, and salt. Integrate the cold butter by rubbing it into the flour mixture with your fingertips until the texture resembles coarse crumbs with visible pea-sized butter bits. Pour in the buttermilk and stir gently with a rubber spatula just until a sticky, uneven dough forms—avoid overmixing.

With lightly floured hands, shape the dough into 8 uneven balls, each slightly larger than a golf ball but smaller than a tennis ball. Arrange them on the prepared sheet, and sprinkle the tops with turbinado sugar. Bake for 13–15 minutes, or until golden brown.

Place one-third of the strawberries in a medium bowl and mash them with a fork or potato masher into a chunky purée. Stir in the remaining sliced strawberries, raspberry jam, and sugar. Allow to sit at room temperature for about 10 minutes to become saucy.

In the bowl of a stand mixer fitted with a whisk attachment—or using hand beaters—whip the cream until soft peaks begin to form. Add the sugar and continue beating just until soft peaks return, being cautious not to overwhip. If preparing by hand, use a deep mixing bowl and a whisk.

To serve, halve each biscuit horizontally using a serrated knife and place the bottom halves onto dessert plates. Distribute about three-quarters of the berry mixture and its juice across the biscuit bases, allowing some to spill over naturally. Add a substantial spoonful of whipped cream, then cap with the biscuit tops. Garnish with remaining berries and a final flourish of cream. Serve without delay.