Preheat oven to 400°F. Line a baking sheet with parchment paper.
In a large bowl, whisk together flour, white sugar, baking powder, and salt. Add lemon zest and stir to combine.
Cut in the cold, cubed butter using a fork or pastry cutter until the mixture resembles coarse crumbs. (If using a stand mixer, use the paddle attachment on the lowest setting.)
In a separate bowl, whisk together the heavy cream, sour cream, egg, and vanilla extract.
Slowly pour the wet ingredients into the dry ingredients, stirring until just combined. Do not overmix.
Gently fold in the blueberries and raspberries, stirring until just incorporated. The berries may break apart slightly - that's normal and okay.
Turn the dough out onto a well-floured surface. With floured hands, pat the dough into a rectangle approximately 1/2 inch thick.
Cut the rectangle into 9 even squares. Cut each square diagonally to create 18 mini triangle scones.
Transfer to the prepared baking sheet.
Brush the top of each scone lightly with heavy cream.
Bake for 14–16 minutes, or until just golden brown. Allow to cool before glazing.