Watermelon Bombe

Line a 3-quart metal mixing bowl with plastic wrap, leaving the edges overhanging. Place the lined bowl in the freezer for 10 minutes to chill; this will serve as the mold. Chill the bowl of an electric mixer as well.

Place the lime sherbet in the chilled mixer bowl. Using the paddle attachment, beat on low speed until the sherbet is spreadable, about 30 seconds. Remove the chilled mold from the freezer. Using a rubber spatula, coat the inside of the mold with all the lime sherbet, making a 1/2-inch-thick layer. Return the mold and the empty mixer bowl to the freezer for 30 minutes.

Place the vanilla ice cream in the chilled mixer bowl. Beat on low speed until spreadable, about 30 seconds. Remove the mold from the freezer. Using the rubber spatula, coat the lime layer with the vanilla ice cream, making a 1/4-inch-thick layer. Return the mold and empty mixer bowl to the freezer for 30 minutes.

Place sugar and 3/4 cup water in a small saucepan. Bring to a boil over medium heat, stirring occasionally. When sugar has dissolved, remove pan from heat.

Place the lime sherbet (referred to as sorbet) in the chilled mixer bowl. Beat on low speed until the sherbet has softened, about 1 minute. Fold in the wafer pieces (or mini chocolate chips). Remove the mold from the freezer. Pack the sherbet mixture into the center, making an even layer 1/3-inch below the ice cream layer. Cover the sherbet portion with sponge cake; trim the cake if necessary. Return the mold to the freezer until completely hardened.

To serve, place a serving plate upside-down on top of the mold. Holding the plate against the mold, invert both. Place a hot, wet kitchen towel over the mold. Remove the metal bowl. Immediately before serving, remove the plastic wrap.