Martha demonstrates how to make a watermelon bombe.
Line a 3-quart metal mixing bowl with plastic wrap, and leave the edges overhanging. Place lined bowl in freezer for 10 minutes; this will be the mold. Chill the bowl of an electric mixer as well.
Place pistachio ice cream in the chilled mixer bowl. Using the paddle attachment of the mixer, beat on low speed until ice cream is spreadable, about 30 seconds. Remove chilled mold from freezer. Using a rubber spatula, coat inside of mold with all the pistachio ice cream, making a 1/2-inch-thick layer. Return mold and clean mixer bowl to the freezer for 30 minutes.
Place vanilla ice cream in chilled mixer bowl. Beat on low speed until spreadable, about 30 seconds. Remove mold from freezer. Using the rubber spatula, coat pistachio layer with the vanilla ice cream, making a 1/4-inch-thick layer. Return mold and clean mixer bowl to freezer for 30 minutes.
Place sorbet in chilled mixer bowl. Beat on low speed until sorbet has softened, about 1 minute. Fold in wafer pieces. Remove mold from freezer. Pack sorbet into center, making an even layer that comes to 1/3 inch below the top of the ice cream layer. Cover sorbet portion with sponge cake; trim cake if necessary. Return mold to freezer until completely hardened.
To serve, place a serving plate upside down on top of the mold. Holding plate against mold, invert both. Place a hot wet kitchen towel over the mold. Remove the metal bowl. Immediately before serving, remove the plastic wrap.