Pulse 2 cups flour, 4 teaspoons baking powder and 1 teaspoon each sugar and kosher salt in a food processor. Pulse in 6 tablespoons diced cold unsalted butter and 3 sliced scallions until the butter is in pea-size pieces. Pulse in 1 cup sour cream. Form into a 4-by-8-inch rectangle. Cut into eight 2-inch squares. Bake on a baking sheet at 425˚ until golden, 12 to 15 minutes.