In a small bowl, cut butter into brown sugar until crumbly. Stir in rhubarb. Spoon evenly into 12 well-greased muffin cups; set aside.
In a large bowl, cream butter and sugar until light and fluffy. Beat in egg. Combine the flour, baking powder, salt and nutmeg; add to creamed mixture alternately with milk and mix well.
Spoon over rhubarb mixture, filling cups three-fourths full. Bake at 350° until a toothpick comes out clean, 15-20 minutes. Cool for 5 minutes before inverting onto a serving plate. Serve warm.
