1. Preheat the oven to 400°F and line a baking sheet with parchment paper.

2. In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.

3. Cut the cold butter into the flour mixture until it looks like coarse crumbs with a few pea-sized pieces remaining.

4. In a separate small bowl, whisk together the heavy cream, egg, and 1 teaspoon vanilla extract.

5. Pour the wet mixture into the dry ingredients and stir gently until the dough just starts to come together.

6. Fold in the blueberries, raspberries, and blackberries carefully so they do not get crushed too much.

7. Turn the dough onto a lightly floured surface and pat it into a round disk about 1 inch thick.

8. Cut the dough into 8 wedges and place them on the prepared baking sheet.

9. Bake for 18 to 22 minutes, until the tops are lightly golden and the centers are baked through.

10. Let the scones cool for 10 minutes on the pan, then move them to a wire rack.

11. Whisk together the powdered sugar, milk or heavy cream, and 1/2 teaspoon vanilla extract until smooth.

12. Drizzle the glaze over the slightly warm or cooled scones and let it set before serving.