Preheat oven to 375°F (190°C).
Spread rhubarb on a parchment-lined tray and sprinkle with 1 cup sugar.
Roast for 20–25 minutes until soft and syrupy. Let cool and reserve some syrup for glaze.
In a large bowl, whisk flour, sugar, baking powder, baking soda, and salt.
Cut in cold butter with a pastry cutter or fork until mixture resembles coarse crumbs.
In a separate bowl, whisk buttermilk and egg together, then stir into flour mixture until just combined.
Gently fold in 1½ to 2 cups of roasted rhubarb (drain excess juice).
Turn dough onto floured surface, pat into a 1-inch thick round, and cut into wedges or circles.
Place scones on parchment-lined baking sheet. Bake at 375°F for 18–22 minutes or until golden. Cool on wire rack.
Mix icing sugar with a few teaspoons of rhubarb syrup and cream to reach drizzling consistency. Drizzle over cooled scones.
