Preheat the oven to 350 degrees and line a muffin tin with 12 liners.
In a large mixing bowl whisk dry ingredients. Add the rhubarb and gently coat with dry mixture. Set aside.
In another mixing bowl combine melted butter (cooled), eggs, sourdough starter, Greek yogurt, and vanilla. Be sure the melted butter isn’t hot or else it will cook the eggs.
Add the wet ingredients into the dry and mix until just combined. Do not over mix. The batter is thick, but don’t worry!
Spoon the muffin batter into each muffin liner and tap the muffin tin to settle the batter.
Mix the crumb toppings with a fork and sprinkle onto each muffin. Gently press into the batter until it sticks well.
Bake muffins for 30-35 minutes. Check with a toothpick. The muffins are done when the toothpick comes out clean. Allow to cool in the tin for a few minutes and then remove to a wire rack to finish cooling. Enjoy warm or completely cooled … you’ll love them either way!