16 tablespoons unsalted butter (2 sticks), frozen
1 1/2 cups fresh blueberries
1/2 cup whole milk
1/2 cup sour cream
2 cups unbleached all-purpose flour, plus additional for work surface
1/2 cup sugar, plus 1 tablespoon for sprinkling
2 teaspoons baking powder
1/4 teaspoon baking soda
1/2 teaspoon salt
1 teaspoon grated lemon zest
Heat oven to 425 degrees.
Whisk flour, 1/2 cup sugar, baking powder, baking soda, salt, and lemon zest in medium bowl, refrigerate until needed. Whisk together milk and sour cream in medium bowl; refrigerate until needed. Place blueberries in freezer until needed.
Remove half of wrapper from each stick of frozen butter. Using the wrapped half as a nice handle, grate unwrapped ends on large holes of box grater (8T.). Melt 2T. of ungrated butter and set aside. You will not be using the remaining 6T. for this recipe.
Add frozen butter to flour mixture and toss whisk or pastry cutter (no hands in flour if possible) until thoroughly coated. Work quickly so you don’t warm the butter any more than necessary.
Add milk mixture to flour mixture; fold with spatula until just combined. Transfer dough to liberally floured work surface. Dust surface of dough with flour; with floured hands, knead dough 6 to 8 times, until it just holds together in ragged ball, adding flour as needed to prevent sticking. Kneed only 6-8 times or the dough will be too warmed by your hands.
Quickly roll dough into a 12-inch square. Fold dough into thirds like a letter, using bench scraper or metal spatula to release dough if it sticks to counter top. Lift short ends of dough and fold into thirds again to form a 4-inch square. Transfer dough to plate lightly dusted with flour and chill in freezer 5 minutes. You mustn’t let it freeze so watch the clock.
Transfer dough to floured work surface and roll into a 12-inch square again. Quickly sprinkle blueberries evenly over surface of dough, then press down so they are slightly embedded in dough. Using bench scraper or thin metal spatula, loosen dough from work surface. Roll dough, pressing to form tight log. Lay seam-side down and press log into 12 by 4-inch rectangle. Using sharp, floured knife, cut rectangle crosswise into 4 equal rectangles. Cut each rectangle diagonally to form 2 triangles and transfer to parchment-lined baking sheet. May cut each triangle in half again for smaller scones.
Brush tops with melted butter and sprinkle with remaining tablespoon sugar. Place on a baking sheet lined with ungreased parchment paper. Bake until tops and bottoms are golden brown, 18 to 25 minutes. Transfer to wire rack and let cool 10 minutes before serving.