Preheat your oven to 350°F (175°C) and prepare your muffin tin by greasing each cup.

In a mixing bowl, combine the rhubarb, 1/4 cup of softened butter, and 3/4 cup of brown sugar. Stir until well coated and divide among the muffin cups.

In another bowl, whisk together the flour, baking powder, salt, and nutmeg. Set aside.

In a third bowl, whisk the granulated sugar, melted butter, and egg until frothy. Gradually stir in the milk and vanilla extract until just combined.

Gently fold the dry mixture into the wet mixture until no flour streaks remain, but do not overmix.

Spoon the batter over the rhubarb base, filling the cups about 2/3 to 3/4 full. Smooth the tops.

Bake for 20–25 minutes until golden brown and a toothpick comes out clean.

Let the muffins cool in the pan for 2-3 minutes, then invert onto a wire rack to release completely.