Step 1: Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
Step 2: In a medium bowl, toss sliced strawberries and blueberries with 2 tablespoons of sugar. Let sit at room temperature to macerate.
Step 3: In a large bowl, whisk together flour, 1/4 cup sugar, baking powder, and salt.
Step 4: Cut the cold butter into the dry ingredients using a pastry blender until the mixture resembles coarse crumbs.
Step 5: Whisk together 2/3 cup heavy cream and the egg, then pour into the flour mixture. Stir until just combined.
Step 6: Turn dough onto a floured surface, pat into a 1-inch thick circle, and cut into 8 rounds using a biscuit cutter.
Step 7: Place rounds on the baking sheet, brush with cream, and bake for 12-15 minutes until golden brown.
Step 8: Whip the remaining 1 cup of heavy cream with vanilla until soft peaks form. Split warm biscuits, fill with fruit and cream, and serve immediately.