1. To make the biscuits, preheat the oven to 400ºF (200ºC). Line a baking sheet with parchment paper or a silicone baking mat.
2. Mix the flour, baking powder, salt, and sugar in the bowl of a stand mixer fitted with a paddle attachment. (It can also be made in a large bowl using a pastry blender.) Add the butter and mix on low speed until the pieces of butter are the size of large kernels of corn. Add the cream or buttermilk and mix until the dough just comes together
3. On a lightly floured counter, roll the dough until it’s 3/4-inch (2cm) thick and with a 2 1/2-inch (8cm) biscuit cutter, cut out six individual biscuits, dipping the cutter in flour between cutting each biscuit. You can gather the scraps and re-roll to cut out a few more biscuits. Put the biscuits on the baking sheet evenly spaced apart. Brush just the tops of the biscuits with the glaze, sprinkle generously with extra sugar, and bake until the tops and sides are browned, about 15 minutes. Remove from oven and let cool.
4. Put half the strawberries in a medium bowl with 1 1/2 tablespoons of sugar and 1 teaspoon kirsch, if using. Use your hands to mash everything together until the berries are juicy. Set aside for at least 30 minutes. You can stir the berries a few times as they sit, which will encourage them to release more of their juices.
5. Slice or quarter the remaining strawberries and mix in another bowl with the raspberries, blueberries, and remaining 1 1/2 tablespoons of sugar, and 1 teaspoon of kirsch, if using.
6. Whip the cream in the bowl of a stand mixer fitted with the whip attachment, or by hand with a whisk until it begins to get stiff, then whip in the sugar and vanilla extract and continue to whip until the cream holds its shape.
7. To assemble the shortcakes, cut each biscuit in half crosswise and place the bottoms on six plates. Spoon a generous amount of the mashed berries and their juice over each bottom piece. Put a dollop of whipped cream on top of each biscuit bottom then divide the mixed berries over each serving. Finish by replacing the tops of the biscuits over the shortcakes.