Preheat oven to 375°F (190°C) and line a muffin tin with paper liners.
In a bowl, whisk flour, sugar, baking powder, baking soda, and salt.
In another bowl, beat eggs, oil, buttermilk, and vanilla extract.
Pour wet ingredients into dry ingredients and mix until just combined.
Gently fold in diced rhubarb.
Divide batter evenly among muffin cups, filling about ¾ full.
Sprinkle brown sugar on top if using.
Bake for 20–22 minutes until a toothpick inserted comes out clean.
Cool in the pan for 5 minutes before transferring to a wire rack.
