Preheat your oven to 350°F (175°C) and prepare muffin tins by greasing them or lining with paper cups.

In a bowl, combine sliced rhubarb with softened butter and brown sugar. Divide the mixture evenly among the muffin cups.

Whisk together flour, baking powder, salt, and nutmeg in a separate bowl.

In another bowl, blend melted butter, brown sugar, and egg until pale. Stir in milk and vanilla extract.

Gently fold the dry ingredients into the wet mixture until just combined.

Spoon the batter over the rhubarb in each muffin cup, filling about 2/3 to 3/4 full.

Bake in the preheated oven for 20-25 minutes, or until golden brown and a toothpick comes out clean.

Let the muffins cool in the tins for 2-3 minutes, then invert onto a cooling rack.