Coffee Cake Scones

Place a rack in middle of oven and preheat to 375°. Whisk ½ cup (63 g) all-purpose flour, ¼ cup (packed; 50 g) dark brown sugar, 1 tsp. ground cinnamon, ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, and ⅛ tsp. baking powder in a small bowl to combine. Using your fingers, rub 4 Tbsp. unsalted butter, cut into small pieces, softened at room temperature 20 minutes (it should be pliable but still somewhat firm), into dry ingredients until largest pieces are pea size. Cover streusel and chill until ready to use.

Stir remaining 2 Tbsp. (25 g) dark brown sugar, 1 tsp. ground cinnamon, and remaining ¼ tsp. Diamond Crystal or Morton kosher salt in another small bowl; set cinnamon sugar aside.

Whisk 1 large egg yolk and 1¼ cups sour cream in a small bowl to combine. Whisk together 3 cups (375 g) all-purpose flour, ½ cup (100 g) granulated sugar, 1 Tbsp. baking powder, and 2½ tsp. Diamond Crystal or 1½ tsp. Morton kosher salt in a large bowl. Using your fingers, rub ½ cup (1 stick) unsalted butter, cut into small pieces, softened at room temperature 20 minutes (it should be pliable but still somewhat firm), into dry ingredients until there are no visible pieces left. Create a well in center of bowl and add egg yolk mixture. Using a rubber spatula, mix until a shaggy dough forms and no dry pockets of flour remain. (You may find it easier to finish mixing with your hands.) Knead dough lightly a few times in bowl to bring together.

Transfer dough to a surface; divide in half and shape each half into a round. Pat rounds down to make ½"-thick disks, then lightly brush tops with water. Sprinkle reserved cinnamon sugar evenly over just one of the rounds to cover surface. Place second round, wet side down, on top and press gently to adhere. Press down on disk to make 1" thick.

Lightly brush top of disk with water. Using a sharp knife, cut into 8 wedges. Arrange scones on a parchment-lined baking sheet, spacing about 2" apart. Sprinkle reserved streusel over scones, dividing evenly and pressing gently to adhere. (Try to keep streusel in large clumps. There might be a little left over; discard or bake separately as a cook’s treat.) Bake scones until bottoms and streusel are golden brown, 20–25 minutes. Do Ahead: Scones can be made 1 day ahead. Store airtight at room temperature.