Preheat your oven to 350°F (175°C) and gather your baking equipment.

In a mixing bowl, combine sliced rhubarb, softened butter, and brown sugar. Press into the bottom of 12 greased muffin cups.

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

In a separate bowl, mix melted butter, granulated sugar, and egg. Whisk until pale, then add milk and vanilla.

Gently combine the dry mixture with the wet mixture until just mixed; the batter should remain slightly lumpy.

Fill each muffin cup with the batter, covering the rhubarb mixture.

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

Cool muffins in the pan for 2-3 minutes, then invert onto a wire rack.

Serve warm for the best experience with a cup of coffee or tea.