Preheat your oven to 400°F (200°C) and grease or line a muffin pan.

In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.

In a large bowl, mix brown sugar and vegetable oil, then add egg, buttermilk, and vanilla extract. Stir until smooth.

Fold the dry ingredients into the wet mixture until just moistened, being careful not to overmix.

Gently fold in larger chunks of rhubarb, reserving some for topping.

Fill each muffin cup about three-quarters full with batter, and top with reserved rhubarb and turbinado sugar.

Bake for 18-22 minutes, checking for doneness with a toothpick.

Cool muffins in the pan for 3-5 minutes before transferring them to a wire rack.