Heat oven to 400°F. Line a baking sheet with parchment.
Whisk flour, sugar, baking powder, baking soda, and salt in a large bowl.
Cut in the cold butter until pea-sized crumbs form. Use a pastry cutter or your fingertips.
Stir buttermilk and vanilla together. Drizzle over the dry mix and fold just until a shaggy dough forms.
Turn dough onto a lightly floured surface. Gently pat into a 7-inch round, about 1 inch thick.
Slice into 8 wedges. Transfer to the prepared sheet, spacing them slightly apart.
Brush tops with a little buttermilk and sprinkle with coarse sugar.
Bake 14–16 minutes, until risen and golden. Cool 10 minutes before serving warm.