Preheat the oven to 350°F (175°C) and grease a muffin tin.

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

In another bowl, mix the wet ingredients: melted butter, buttermilk, egg, and vanilla extract until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined.

Fold in the chopped rhubarb gently.

Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.