Preheat the oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners.

In a large bowl, cream the softened butter and granulated sugar until pale and fluffy, about 3 to 4 minutes.

Add the eggs one at a time, mixing well after each. Stir in the vanilla.

In a separate bowl, whisk together the flour, baking powder, and salt.

Add the dry mixture to the wet ingredients in three parts, alternating with milk, beginning and ending with the dry mix. Mix until just combined; do not overmix.

Fold the chopped rhubarb into the batter gently so the pieces stay evenly distributed.

Fill each muffin cup about two-thirds full.

Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Let muffins cool in the tin for a few minutes, then transfer to a wire rack to cool completely.