Cook the rhubarb: In a small saucepan, combine the rhubarb, sugar, and a splash of water. Cook over medium heat, stirring occasionally, for 5–7 minutes until the rhubarb softens and thickens slightly. Let cool.
Mix dry ingredients: In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
Add wet ingredients: Stir in sourdough discard, milk, egg, melted butter, and vanilla extract until just combined.
Fold in rhubarb: Gently fold in the cooled rhubarb mixture. Don’t overmix—swirls are fine.
Preheat skillet: Heat a cast iron or non-stick skillet over medium heat and grease lightly with butter.
Cook pancakes: Scoop ¼ cup batter per pancake into the skillet. Cook until bubbles form and the edges look set, 2–3 minutes. Flip and cook the other side for 1–2 minutes until golden brown.
Serve: Enjoy warm with maple syrup, butter, or more stewed rhubarb on top.