First, move the oven rack to the middle position and preheat the oven to 425° Line a baking sheet with a silicone baking mat or parchment paper; set aside.
Then, using the large holes of a box grater, grate the entire stick of frozen butter, and then freeze the grated butter and fresh blueberries (if you’re using fresh berries) for at least 30 minutes before using them in the recipe.
In a liquid measuring cup, whisk together milk and sour cream; set aside.
Next, in a medium bowl, whisk together flour, sugar, zest, baking powder, baking soda, and salt. Sprinkle the frozen grated butter over the flour mixture, and gently toss in the flour until the grated butter pieces are covered in flour.
Then, pour the milk mixture over the flour mixture, and use a rubber scraper to gently fold until the dough comes together (be careful not to over-mix!).
Next, move the dough to a floured countertop or work surface. Sprinkle the top of the dough and your hands with flour and knead for 30 seconds, or until the dough comes together and isn’t too sticky.
Use a rolling pin to roll the dough into a 12-inch square.
Use a bench scraper to remove the dough from the countertop and fold the dough into thirds.
Lift up the short ends of the dough to fold into thirds again (you should have about a 4-inch square stacked dough in front of you). Wrap loosely in plastic wrap and freeze for 10 minutes.
Then, place the dough on a floured work surface and roll into a 12-inch square again. Distribute the blueberries over the surface of the dough, and press them down slightly. Use the bench scraper again to loosen the dough from the work surface.
Press the dough while your roll it to form a tight log. You should have a 12x4-inch rectangle of rolled dough. Cut the dough into 4 equal pieces and then cut each piece into 2 triangles. You should have 8 triangles of dough.
TO FINISH:
Place the triangles on the prepared baking sheet. Brush the dough with melted butter and sprinkle with sugar.
Finally, bake for 18-24 minutes, or until the tops and bottoms are golden brown. Transfer the scones to a wire rack and cool at least 15 minutes (if you can wait that long) before serving.