Preheat oven to 375°F (190°C). Line a muffin tin with paper liners or grease lightly.

In a large bowl, whisk together the flour, sugars, baking powder, baking soda, salt, and cinnamon.

In a separate bowl, whisk together the buttermilk, oil, egg, and vanilla extract until combined.

Add the wet ingredients to the dry ingredients and stir just until combined—do not overmix.

Gently fold in the chopped rhubarb.

Divide batter evenly among the muffin cups, filling about 3/4 full.

In a small bowl, mix together crumb topping ingredients and sprinkle evenly over each muffin.

Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.

Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.