In a small bowl, whisk together the egg, sour cream, and lavender extract. Set the mixture in the fridge.
In a medium-sized mixing bowl, add the flour, baking powder, salt, and sugar. Cube up your cold butter stick and add it into the dry ingredients.
Using a pastry cutter or your fingers, cut the butter into the flour mixture until the butter pieces are quite small.
Make a well in the center of the ingredients, then pour in the sour cream/egg mixture. Lightly mix the ingredients together, then prep your berries.
Place a fine mesh sieve over a bowl, and pour the blackberries into the sieve. Lightly press on the blackberries (but do not mash them), to get most of the macerated berry juice off of them. Save the juice for the glaze.
Add in the blackberries. Use your hands to lightly knead the scone dough together until it comes together.
Form the dough into a circle that is 6" in diameter. Cover the dough and let it rest in the freezer for at least 30 minutes, up to overnight.
Preheat the oven to 425°F / 220°C. Line a cookie sheet with parchment paper or a silicone baking mat.
Slice the scones into 8 equally sized triangles. Evenly space them apart on the lined baking sheet.
If desired, you can brush a little bit of heavy cream on top of the scones to encourage the tops to brown. Bake for 20-25 minutes, or until the scones are golden brown on top.