Pre heat the oven to 400 degrees Fahrenheit with the rack in the middle. Line a baking sheet with parchment paper.

In the bowl of a food processor, pulse the dry ingredients until well combined.

Add in the mashed sweet potato, buttermilk, egg and vanilla. Mix until the dough just starts pulling away from the sides of the bowl.

Add in the butter, pulse until it looks like coarse crumbs.

Transfer the dough to a lightly floured surface. Pat into an 8 inch circle with lightly floured hands. Cut into 8 wedges. Brush the tops with the egg yolk and sprinkle with some sugar.

Bake for 18-21 minutes or until golden brown. Transfer to wire racks, cool for 5 minutes and serve warm.