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

In a large bowl, whisk together the flour, 1/3 cup sugar, baking powder, and salt.

Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Stir in the finely chopped rhubarb.

Gradually pour in the cold heavy cream, mixing gently with a fork until just combined. Do not overmix; the dough will be shaggy.

Turn the dough out onto a lightly floured surface. Gently pat or lightly roll the dough into a 3/4-inch thick circle (about 7 inches in diameter). Cut into 8 equal wedges.

Place the wedges on the prepared baking sheet. Brush the tops lightly with a little extra cream and sprinkle with the remaining 1 tablespoon of granulated sugar.

Bake for 14 to 16 minutes, or until the tops are golden brown. Let cool slightly on a wire rack.

If using the glaze, whisk the powdered sugar and milk together until smooth and drizzle over the warm scones before serving.