Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

In a large bowl, whisk together the flour, sugar, baking powder, and salt. Cut in the cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining.

Gently toss the chopped rhubarb into the flour mixture, ensuring it is evenly coated. Slowly pour in the 1/2 cup of heavy cream, mixing with a fork just until a shaggy dough forms. Do not overmix.

Turn the dough out onto a lightly floured surface. Gently knead 3-4 times, just enough to bring it together. Pat or roll the dough into a 3/4-inch thick circle or rectangle. Use a 2.5-inch biscuit cutter or knife to cut out 8 wedges or rounds.

Place the scones onto the prepared baking sheet. Brush the tops lightly with extra heavy cream and sprinkle with a tiny bit of granulated sugar (optional). Bake for 16-18 minutes, or until golden brown and cooked through.

While the scones cool slightly, prepare the glaze: In a small bowl, whisk together the powdered sugar, sour cream, and vanilla extract until smooth. Add milk or cream, one teaspoon at a time, if the glaze is too thick.

Drizzle the sour cream glaze generously over the warm (or fully cooled) scones before serving.