Line a large baking sheet with parchment paper and set aside. Slice the rhubarb stalks into 1/4" pieces and toss them in a small bowl with 3 tablespoons of the sugar.

Whisk the flour, baking powder, remaining sugar and salt together in a large bowl. Cut the butter into the flour mixture and then mix using a pastry blender or a large fork until the mixture resembles coarse crumbs. Add the rhubarb to the flour mixture and toss to coat them in flour.

Add the buttermilk, then gently fold until the dough starts to come together. Transfer the dough to a floured surface and gently bring the dough together with lightly floured hands. Cut the dough in half and then shape and flatten each piece into a disc about 1-1.5 inches high. Cut each circle into 6 scones using a sharp knife or bench scraper. Arrange the scones on the baking sheet and place in the freezer for 15 minutes.

While the scones are in the freezer, preheat the oven to 400 degrees F. Bake for 20-25 minutes or until golden brown on the top.