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 2 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.