Preheat oven to 400°F (200°C) and line a baking sheet.
In a bowl, whisk flour, sugar, baking powder, and salt.
Cut in cold butter until mixture resembles coarse crumbs.
Fold in rhubarb.
Whisk cream, egg, and vanilla; stir into dry ingredients just until dough forms.
Pat dough into a circle about 1 inch thick.
Cut into 8 wedges and place on baking sheet.
Brush tops with a little cream and sprinkle with sugar if desired.
Bake 18–22 minutes until golden.
Cool slightly and drizzle with glaze if using.