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

In a large bowl, whisk together the flour, 1/4 cup granulated sugar, baking powder, and salt.

Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. This keeps the scones flaky.

Gently fold in the chopped rhubarb until evenly distributed. Create a well in the center and pour in the 3/4 cup of cold heavy cream. Mix with a fork just until the dough barely comes together; do not overmix.

Turn the shaggy dough out onto a lightly floured surface. Gently knead 3 or 4 times only to bring it together. Pat the dough into a 3/4-inch thick circle. Use a 2.5-inch round cutter (or knife to cut into 8 wedges) to cut out the scones.

Place the cut scones onto the prepared baking sheet. Brush the tops lightly with the 2 tablespoons of extra heavy cream and sprinkle generously with coarse sparkling sugar.

Bake for 15 to 18 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean. Let cool on the baking sheet for a few minutes before transferring to a wire rack.