Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large bowl, combine the flour, sugar, baking powder, and salt.
In another bowl, whisk together melted butter, eggs, vanilla extract, and milk.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the chopped rhubarb until evenly distributed.
Fill each muffin cup about two-thirds full with the batter.
Bake in the preheated oven for 20-25 minutes or until a toothpick comes out clean.
Remove from the oven and let the muffins cool in the pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely.
Dust with powdered sugar before serving, if desired.