Preheat oven to 350 degrees. Line a rimmed baking sheet with foil; set aside.
RHUBARB: Place rhubarb in a buttered 10-inch springform pan. In a mixing bowl, whisk together sugar, flour and nutmeg; sprinkle over rhubarb. Drizzle with melted butter; set aside.
CAKE BATTER: In a large bowl, beat the sugar and melted butter until well combined. Beat in the egg.
In a separate bowl, whisk together flour, baking powder, nutmeg and salt. Gradually add flour mixture alternately with milk, beating well after each addition. Dollop batter over the rhubarb mixture, spread with an offset spatula.
Place on the prepared baking sheet and bake for 35-40 minutes or until a toothpick inserted in the center comes out clean. Loosen edges immediately and invert onto a serving dish. Serve warm topped with whipped cream if desired.