This ice cream cake is sure to be a hit with kids and adults alike. You can add the sundae toppings before serving, or put them out in bowls to let each person personalize his or her own slice.
Preheat the oven to 350ºF. Line a 9-by-13-inch cake pan with aluminum foil, and coat it with cooking spray.
Beat the cake mix, oil, egg and 1/4 cup water in a large bowl with an electric mixer on medium speed until smooth. Let stand a few minutes until no longer tacky and then pat into the prepared pan.
Bake until the edges are set, the cake has deflated and the top is no longer wet and shiny, 20 to 25 minutes. Let cool completely.
Spread the softened ice cream over the cake in 1 even layer. Cover with plastic wrap and freeze until firm, at least 4 hours or overnight.
Let the cake stand at room temperature for about 20 minutes before serving. Whip the cream with the sugar in a chilled medium bowl with an electric mixer until stiff peaks form. Spread over the ice cream. Top with your favorite sundae toppings, such as those suggested above. Use the foil to lift the cake out of the pan and place on serving platter. Slice and serve.