Rhubarb Custard Cake

1 package (18-1/4 ounces) yellow cake mix
4 cups chopped fresh or frozen rhubarb
1 cup sugar
1 cup whipping cream
Whipped cream and fresh mint, optional

Prepare cake batter according to package directions. Pour into a greased 13-inch x 9-inch x 2-inch baking dish. Sprinkle with rhubarb and sugar. Slowly pour cream over top. Bake at 350 degrees for 40 to45 minutes or until golden brown. Cool for 15 minutes before serving. Garnish with whipped cream and mint if desired. Refrigerate leftovers.