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

In a large bowl, whisk together the flour, 1/4 cup 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 butter pieces remaining.

In a separate small bowl, lightly beat the egg with the heavy cream. Pour this liquid mixture into the dry ingredients, stirring just until a shaggy dough forms. Do not overmix.

Gently fold in the chopped rhubarb that has been tossed with 2 tablespoons of sugar and the vanilla extract. Be careful not to crush the rhubarb.

Turn the dough out onto a lightly floured surface. Gently pat or knead (no more than 3-4 times) into a 3/4-inch thick circle or rectangle. Cut into 8 wedges or use a biscuit cutter.

Place the scones onto the prepared baking sheet. Brush the tops lightly with any remaining heavy cream for better browning.

Bake for 16-18 minutes, or until the tops are golden brown and a toothpick inserted comes out clean. Let cool slightly on a wire rack.

If using the glaze, whisk the powdered sugar and milk/lemon juice until smooth. Drizzle over the warm scones before serving.