Combine melted butter, rhubarb and brown sugar in a small bowl and mix well. Place in 12 greased muffin cups. Beat together butter, sugar and eggs until fluffy. Combine flour, baking powder, salt and nutmeg and add to creamed mixture alternately with milk. Stir just to moisten, then spoon on top of rhubarb mixture. Bake at 350 degrees for 20 to 25 minutes. Invert on cooling rack and leave pan over muffins for a few minutes so all the rhubarb runs out. Serve warm.