1. Put oven rack in middle position and preheat oven to 450°F.
2. Whisk together flour, baking powder, and salt. Blend in butter with your fingertips until mixture resembles coarse meal. Add scallions and 3/4 cup milk and stir with a fork just until a dough forms.
3. Turn out dough onto a lightly floured surface and knead 6 to 8 times, then pat into an 8-inch square (1/2 inch thick). Cut into 16 (2-inch) squares and transfer to a buttered baking sheet, about 1 1/2 inches apart. Brush with remaining tablespoon milk, then bake until golden, 12 to 15 minutes.