For the cake: Position an oven rack in the middle of the oven and preheat to 350 degrees F. 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, about 1 hour.
Spread the softened ice cream over the cake in an even layer. Cover with plastic wrap and freeze until firm, at least 4 hours or overnight.
For the chocolate sauce: Combine the chocolate, cream, corn syrup, butter and salt in a medium saucepan over medium heat. Whisk periodically as the mixture comes to a gentle simmer. Simmer for 5 minutes, stirring occasionally. If the sauce starts to boil, turn down the heat. Cool 15 minutes before serving. Transfer to a bowl or small pitcher.
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 the whipped cream over the ice cream layer to cover.
Use the foil to lift the cake out of the pan and place on a serving platter. Slice and serve along with the chocolate sauce and your favorite toppings, such as chocolate sprinkles, chocolate chips, crushed chocolate wafer cookies, maraschino cherries and chocolate-covered raisins, nuts and pretzels.
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