Preheat your oven to 375°F (190°C) and prepare a muffin tin by lining it with paper liners or greasing each cup.

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

In a separate bowl, mix together the egg, sugar, vegetable oil, vanilla, and buttermilk until smooth.

Gently combine the wet and dry ingredients until just mixed, avoiding overmixing.

Fold in the rhubarb carefully to ensure it's evenly distributed.

Divide the batter among the muffin cups, filling each about ¾ full.

Bake for 18 to 22 minutes until the muffins are golden brown and a toothpick comes out clean.

Let the muffins cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.