01 -
Heat your oven to 350°F and line 24 muffin cups with paper liners or give them a light coating of cooking spray.
02 -
In a large mixing bowl, whisk together both sugars, vegetable oil, buttermilk, egg, and vanilla until everything is well combined and smooth.
03 -
Gently fold in the flour and baking soda, stirring just until the mixture comes together. Don't overmix - a few lumps are perfectly fine.
04 -
Add the diced rhubarb to the batter and gently stir until the pieces are evenly distributed throughout. Set the bowl aside while you prepare the topping.
05 -
In a small bowl, combine the sugar, cinnamon, and softened butter. Use a fork to mix everything together until it forms a crumbly mixture.
06 -
Divide the batter evenly among the muffin cups, filling each about 2/3 full. Sprinkle about 1 teaspoon of the cinnamon topping over each muffin.
07 -
Bake for 18-22 minutes, or until the tops are golden brown and a toothpick inserted in the center comes out clean or with just a few moist crumbs.
08 -
Let the muffins cool in the pan for about 15 minutes, then transfer them to a wire rack to cool completely before serving.