Preheat oven to 450°. Line a baking sheet with parchment paper.

In a large bowl, stir together flour, sugar, baking powder and salt. Using a pastry blender or 2 knives, cut in butter until mixture resembles coarse crumbs.

Stir in 2/3 cup buttermilk. Dough will be quite crumbly. Turned out onto a work surface, and knead until dough comes together, about 2 minutes. Roll out to 1-inch thickness. Using a 2 3/4-inch round cutter, cut out 6 circles, rerolling dough as necessary.

Place circles on prepared baking sheet. Brush with remaining 2 tbsp. buttermilk; sprinkle with turbinado sugar. Bake for 12 minutes or until golden brown. Cool on a wire rack.