Place rhubarb in a well-greased 10-in. heavy oven-proof skillet. Combine sugar, arrowroot and nutmeg; sprinkle over rhubarb. Drizzle with butter; set aside.
In a large bowl, combine the flour, baking powder, nutmeg and salt. Add the butter, sugar, egg and milk until well-blended. Spread over rhubarb mixture.
Bake at 350° for 35 minutes or until a toothpick inserted near the center comes out clean. Loosen edges immediately and invert onto a serving dish. I put the serving dish upside down over the baking dish. Firmly holding the 2 parts together, I quickly turn them upside down. Then I let it sit a little to let the cake and rhubarb settle onto the serving dish. Sometimes I need to pull some of the rhubarb off the baking dish and on to the cake.
Serve warm or cold. Serve with whipped cream or ice cream, if desired. Yield: 8-10 servings.