Preheat oven to 375°F (190°C). Line a baking tray with parchment paper.
In a large bowl, whisk together flour, sugar, baking soda, and salt.
In a separate bowl, whisk the melted butter, egg, sour cream, and vanilla.
Fold the wet ingredients into the dry just until combined don’t overmix.
Gently fold in the chopped rhubarb.
Use a spoon or scoop to drop generous mounds of batter onto the tray.
Sprinkle with coarse sugar if desired.
Bake for 12–15 minutes, until edges are golden and centers are set.
Cool slightly on tray, then transfer to a rack. Enjoy warm or at room temp!
