Preheat your oven to 400°F. Oil your muffin pans or insert muffin cup liners.
Mix Rhubarb: In a small bowl, toss the diced rhubarb with 2 tbsp of sugar; set aside. This helps draw out the juices and sweetens the rhubarb.
Combine Dry Ingredients: In a small bowl, whisk the flour (2 c), sugar (1 c), baking powder (1/2 tsp) and salt (1/2 tsp).
Combine Wet Ingredients: In a medium/large bowl, mix the 2 eggs, melted butter (1/2 stick), sour cream (1/2 c), sourdough discard (1/2 c), and vanilla extract (2 tsp). Stir until combined to avoid overmixing, which can make the muffins tough.
Add sugared rhubarb: Fold the sugared rhubarb into the dry mixture.
Combine Mixes: Then, gradually add this mixture to the wet ingredients, stirring by hand just until the flour is incorporated.
Bake: Add the batter to your prepared muffin pans, filling them about 2/3rds full in standard muffin pans or completely full in muffin top pans (as shown in the picture). This recipe made 12 muffin top muffins. Sprinkle the muffins with 1 tbsp of sugar. Bake for about 12-15 minutes, or until the edges are golden and a toothpick (or chopstick) inserted into the center comes out clean.
Cool and enjoy: Transfer the muffins to a wire rack to cool completely.