Preheat your oven to 425 degrees F and line a large baking sheet with parchment paper.
Slice the rhubarb stalks into 1/4" pieces and toss them with 3 tablespoons of the sugar.
Sift the flour, baking powder 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. Stir in 1/3 cup of the sugar and then stir in the rhubarb.
Add the buttermilk and stir and fold until the dough forms. Transfer the dough to a floured surface and bring the dough together with floured hands. Flatten the dough gently into a disk about 1-1.5 inches high. Cut the circle into 8 scones.
Arrange the scones on the baking sheet, and bake 18-20 minutes or until golden brown on top.
1. Optional: Before putting the scones in the oven, brush the top of each with buttermilk and sprinkle with course sugar.
2. This recipe makes 8 large scones, but you can also cut the dough in half when you transfer it to your floured surface, shape each half into a disk and cut each disk into 6-8 scones each for a total of 12-16 scones.