Preheat the oven to 425°F and line a baking sheet with parchment paper.
Whisk together the flour, baking powder, baking soda, salt, and ⅓ cup sugar.
Add the butter and use a pastry blender or fork to blend the ingredients until it resembles coarse sand.
Stir in the buttermilk.
Scoop the dough into balls and arrange them on the prepared baking sheet. Brush lightly with beaten egg and sprinkle with the remaining sugar.
Bake for 15 to 20 minutes, until golden brown. Allow to cool completely.
To serve, beat together the heavy cream and sugar until stiff peaks form.
Cut one biscuit in half and top with strawberries, blueberries and a dollop of whipped cream.
Notes
If you don’t have buttermilk, you can make a substitute by adding 1 tablespoon of lemon juice or vinegar to 1 cup of milk. Let it sit for 5 minutes before using.