Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.

In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In another bowl, mix the buttermilk, vegetable oil, and egg until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined. Fold in the chopped rhubarb.

In a small bowl, combine the flour, brown sugar, softened butter, and cinnamon. Mix until crumbly.

Divide the muffin batter evenly among the muffin cups, filling each about 2/3 full.

Sprinkle the crumbly topping over each muffin.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan for a few minutes before transferring to a wire rack to cool completely.