Preheat your oven to 350°F (175°C).

In a small bowl, combine the sliced rhubarb with 1/4 cup of softened unsalted butter and 1/2 cup of brown sugar. Mix thoroughly until well-coated.

In a medium bowl, whisk together 1.75 cups of all-purpose flour, 2 teaspoons of baking powder, 1/2 teaspoon of salt, and 1/2 teaspoon of freshly grated nutmeg.

In a separate bowl, mix together 1/3 cup of melted unsalted butter, 1/2 cup of granulated sugar, and 1 egg. Whisk until pale and creamy, then add in 1/2 cup of milk and 1/2 teaspoon of vanilla extract.

Fold the dry ingredients into the wet mixture, stirring just until combined.

Distribute the batter over the rhubarb mixture in the muffin cups, filling each cup about 2/3 full.

Bake for 20-25 minutes until the tops are golden brown and a toothpick comes out clean.

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