Preheat oven to 350°F and add muffin liners to 24 muffin cups.
In a large bowl, stir together the sugar, brown sugar, oil, buttermilk, egg and vanilla.
3/4 cup sugar, 3/4 cup brown sugar, 1/2 cup vegetable oil, 1 cup buttermilk*, 1 egg, 1 teaspoon vanilla
Next, add the flour and baking soda and stir until just combined.
2 3/4 cup flour, 1 teaspoon baking soda
Add the rhubarb and stir until well dispersed throughout the batter; set aside.
3 cups diced rhubarb
Next, add the topping ingredients in a small bowl and use a fork to mix until combined.
1/2 cup sugar, 1/2 teaspoon cinnamon, 1 tablespoon butter
Fill each muffin cup 2/3 full and top each muffin with about 1 tsp topping.
Bake for 18-22 minutes.
Let cool for 15 minutes and remove from the muffin tin to cool completely.
Notes
Make Your Own Buttermilk: Add 1 tablespoon white vinegar to a liquid measuring cup and add enough milk to fill to the 1 cup mark. Give it a stir and add it to the recipe!