Grease muffin cups. Using a pastry blender, cut brown sugar into butter until the mixture resembles small pebbles. Combine with rhubarb and spoon into muffin cups.
Cream together sugar, butter, and egg. Combine dry ingredients and add alternately with milk to creamed mixture, stirring just until flour is moistened. Spoon batter over rhubarb mixture, filling cups about half full. Bake 20 minutes. Turn upside down on rack to cool before removing from tins.