2 1/2 cups all-purpose flour (spooned and leveled), plus more for work surface
1/4 cup granulated sugar
1 tablespoon baking powder
1/2 teaspoon coarse salt
1/2 cup (1 stick) cold unsalted butter, cut into small pieces
1/2 cup plus 2 tablespoons buttermilk
1 tablespoon finely grated orange zest plus 1/4 cup orange juice (from 1 orange)
1 large egg yolk
1 1/2 teaspoons poppy seeds
Coarse or sanding sugar, for sprinkling
Butter and fruit jam (optional), for serving
Preheat oven to 400 degrees, with racks in upper and lower thirds. In a large bowl, whisk together flour, granulated sugar, baking powder, and salt. With a pastry blender or your fingertips, work butter into flour mixture until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. With a fork, stir in 1/2 cup buttermilk, orange zest and juice, egg yolk, and poppy seeds until just combined.
On a lightly floured surface, turn out dough and knead several times. Form dough into a 7-inch square; cut into 9 squares, then cut each in half diagonally. Transfer dough triangles to two parchment-lined rimmed baking sheets. Brush tops with 2 tablespoons buttermilk; sprinkle with coarse sugar. Bake until scones are pale golden, 15 to 17 minutes, rotating sheets halfway through. Let cool on wire racks. Serve with butter and jam if desired.
Store scones in an airtight container, up to 3 days. Warm in oven before serving.