Each of this elegant ice-cream dessert's layers must freeze solid before the next is added. Dont rush the freezing time or the layers will run together.
Place a circle of parchment in the bottom of an 8-by-3-inch springform pan. Lightly coat bottom of pan with cooking spray to adhere parchment; set aside. Chill pan in freezer for 30 minutes.
Beat 1 1/2 pints chocolate ice cream in the bowl of an electric mixer fitted with the paddle attachment until soft but still holding its shape, 1 to 2 minutes. Using a small offset spatula or a spoon, spread the ice cream evenly on the bottom and up the sides of the springform pan. Work quickly, or the ice cream will not stay on the sides very long. If the ice cream starts to fall, return ice cream-lined pan to freezer for 10 minutes, and then finish lining. Transfer ice cream-lined pan to freezer until hardened, about 45 minutes.
In the bowl of a food processor combine 2 bananas, milk, 1/4 cup sugar, and vanilla. Process 30 seconds or until smooth, and add remaining banana. Pulse until there are only small pieces of banana, about the size of peas. Transfer mixture to a bowl, and set aside.
Remove the pan from the freezer, and spread half, about 3/4 cup, of the banana mixture into the bottom of the chocolate ice cream-lined pan. Return to the freezer until hardened, about 2 hours. Cover the remaining banana mixture with plastic wrap, and refrigerate. When hardened, remove the pan from the freezer; ladle 1 cup room-temperature Hot Fudge Sauce over the banana layer. Return to the freezer until hardened, about 15 minutes. Set aside the remaining fudge sauce to serve with the finished bombe, or store in a plastic container in the refrigerator for up to 1 week.
Soften vanilla ice cream as above. Remove pan from the freezer, and add 1 cup softened vanilla ice cream, using a small offset spatula or spoon to spread it evenly. Return pan to freezer, and let ice cream harden, about 1 hour. The remaining soft vanilla ice cream can be returned to its container and stored in the freezer; it will soften quickly when you need it again.
Preheat the oven to 350 degrees. Spread the walnuts evenly on a baking pan, and toast them in the oven, about 7 minutes, until fragrant and lightly golden. Remove from the oven, and let cool. In a medium nonstick saute pan, combine remaining 1/4 cup sugar and water. Stir over medium-high heat until sugar is dissolved; stop stirring, and let boil. The sugar will begin to caramelize in the pan; you can pick up the pan and slightly swirl the sugar to allow for even caramelizing. Let the caramel turn to a dark golden brown, about 5 minutes, and immediately add the nuts to the pan. Turn off the heat. Using a wooden spoon, stir the nuts in the caramel to coat them evenly. Transfer the nuts directly onto a Silpat baking mat (a French nonstick baking mat), and spread them into an even layer. Let cool; break into small pieces.
Remove ice cream mold from freezer, and sprinkle an even layer of nuts over vanilla layer. Remove banana mixture from the refrigerator, and spread the remaining 3/4 cup over the nuts. Quickly return mold to the freezer, and let chill. When banana layer has hardened, about 2 hours, resoften the remaining cup of vanilla ice cream and spread evenly with small offset spatula or spoon.
When vanilla ice cream has hardened, soften the remaining cup of chocolate ice cream; remove mold from freezer. Add final layer of chocolate ice cream, spreading it all the way to the edges and smoothing the top. If chocolate ice cream extends above the inside rim of the mold, run your finger around the top of the inside of the rim to give a clean edge. Cover with plastic wrap, and place in freezer to chill, about 4 hours, or overnight. When ready to serve, remove mold from freezer, and dip in very hot water for a few seconds. Unlatch the ring, and gently pull away from the bombe. Using a parchment bottom allows the bombe to slide off the pan base. Transfer bombe to cutting board. Using a sharp knife, slice the bombe into wedges, and serve.