In a bowl, mix flour, brown sugar, baking powder, 1/2 teaspoon cinnamon, and salt. Add 1/2 cup butter and, with a pastry blender or your fingers, cut or rub in until pea-size crumbs form.
In a small bowl, whisk pumpkin and 1/2 cup milk until well blended. Add to flour mixture and stir just until dough is evenly moistened.
Scrape onto a lightly floured board, turn over to coat, and gently knead just until dough comes together, 5 or 6 turns. Pat dough into a 6-inch round 1 1/2 inches thick; cut into 6 equal wedges.
Separate wedges and place on a lightly buttered 12- by 15-inch baking sheet. In a small bowl, beat egg yolk and 1 tablespoon milk to blend; brush lightly over tops of scones (discard any remaining egg wash). In another small bowl, mix granulated sugar and remaining 1/4 teaspoon cinnamon; sprinkle evenly over scones.
Bake in a 375° regular or convection oven until scones are golden brown, 25 to 30 minutes. Transfer to a rack; serve warm or cool.