8 tbsp. (1 stick) unsalted butter, frozen whole
1½ cups (7½ oz.) fresh blueberries
½ cup whole milk
½ cup sour cream
2 cups (10 oz.) all-purpose flour, plus more for dusting the work surface
½ cup sugar, plus extra for sprinkling
2 tsp. baking powder
¼ tsp. baking soda
½ tsp. salt
1 tsp. finely grated lemon zest
2 tbsp. unsalted butter, melted
Adjust an oven rack to middle position and preheat to 425˚ F. Grate the frozen butter on the holes of a large box grater. (I like to use my food processor for this – fast and easy.) Place the blueberries in the freezer until needed.
Whisk together the milk and sour cream in a medium bowl; refrigerate until needed. Combine the flour, ½ cup sugar, baking powder, baking soda, salt and lemon zest in a medium mixing bowl. Whisk to combine. Add the grated butter to the flour mixture and toss with fingers until thoroughly coated.
Add the milk mixture to the dry ingredients and fold with a spatula just until combined. Transfer the dough to a generously floured work surface. Dust the top of the dough with flour, and knead with well floured hands, 6-8 times, just until the dough holds together in a ragged ball. Add small amounts of flour as needed to prevent sticking.
Roll the dough into a 12-inch square. Fold the dough into thirds like a business letter (a dough scraper really helps with these steps). Fold the short ends of the dough into the center in thirds, to form an approximate 4-inch square. Transfer the dough to a plate lightly dusted with flour and chill in the freezer for 5 minutes.
Return the dough to the floured work surface and roll into an approximately 12-inch square again. Sprinkle the blueberries evenly over the surface of the dough, and gently press down so that they are slightly embedded in the dough surface. Using a dough scraper, roll the dough up to form a tight log. Lay the log seam side down and press the the log into a 12 by 4-inch rectangle. Using a sharp, floured knife, cut the rectangle crosswise into 4 equal rectangles. Cut each rectangle diagonally to form 2 triangles. Transfer to a parchment lined baking sheet.
Brush the tops of the scones with melted butter and sprinkle lightly with sugar. (If freezing ahead of time, flash freeze on the baking sheet for 20 minutes, then wrap individually and store in a freezer bag until needed.) Bake until the tops and bottoms are golden brown, 18-25 minutes. Transfer to a wire rack and let cool at least 10 minutes before serving.