For the Cake:
1 cup cake flour (not self rising)
1/2 tsp. baking soda
1/4 teasp. baking powder
1/4 tsp. salt
1/8 tsp. ground cinnamon
1 very ripe banana, mashed
1 very ripe banana, finely chopped
1/4 cup buttermilk
1/2 tsp. vanilla extract
4 T unsalted butter, softened
1/2 cup brown sugar
For Ganache (2nd layer)
12 oz. semisweet chocolate pieces
1 very ripe banana, mashed
1 cup heavy cream
For Ice cream Bombe:
(First Layer)2 quarts vanilla ice cream, softened
(Third Layer) 2 pints pistachio ice cream, softened
(Fourth Layer) 2 pints chocolate icecream, softened
For Hard Chocolate Topping:
8 oz. semisweet chocolate pieces
2 T shortening
1/4 cup chopped pistachios for garnish
2 Days ahead; Make Banana Cake:
Preheat oven to 350º. Butter a 9-inch or larger round cake pan (cake should fit the top of the mold or bowl in which you layer the ice cream). Line with parchment cut to fit, and butter and flour the parchment. Sift flour, baking powder, soda, salt, and cinnamon into a medium bowl, Whisk mashed banana, buttermilk, and vanilla in a small bowl.
Beat butter and sugar, with a mixer on medium speed. Add egg, beat until well combined. Reduce speed to low, and add flour mixture in 2 additions, alternating with banana mixture. Add chopped banana and beat until well combined. Pour batter into pan.
Bake until golden brown and tester comes out clean, about 20 minutes. Let cool on rack in pan for 10 minutes. Invert pan onto rack, remove parchment, and flip cake so top faces up. Let cool completely. If making bombe the next day, wrap cake in plastic and refrigerate until ready to use.
1 day ahead:
Make Ganache Layer next:
Place chocolate and banana in a heat proof bowl. Heat cream in a saucepan until bubbles begin to form around edges. Pour over chocolate and banana, and let stand for one minute. Whisk until combined. Set bowl over a a pot of simmering water, and whisk until chocolate melts. Refrigerate until thick and and spreadable, about 1 hour.
To assemble Bombe:
First Layer: Chill in freezer a 5-quart heatproof bowl (I used a pyrex glass bowl) with a 9 1/2 - inch-diameter rim. Spread softened vanilla ice cream in an even layer on bottom and up sides of bowl. Ice cream should form a concave layer that comes up to the rim of the bowl. Freeze until set, about 20 minutes. Using an offset spatula, smooth the surface of the ice cream layer. You may have to work the ice cream up to the rim as it may slide down the sides at first. Freeze again until very firm, at least 1 hour.
Second Layer: Spread refrigerated ganache mixture in a thin, even layer over the entire surface of the vanilla ice cream. Freeze until firm, about 30 minutes.
Spread pistachio ice cream in a flat, smooth layer in bottom of bowl, and not up sides. Freeze until firm, about 30 minutes.
Spread chocolate ice cream in a flat, smooth layer over pistachio ice cream. All the layers should be even now, and reaching the top. Freeze until firm, about 30 minutes.
Place banana cake over chocolate ice cream, top side up, and press gently until cake fits snugly against ice cream. Trim cake or ice cream to fit. Cover with plastic wrap, and freeze overnight (or up to 3 days).
To unmold bombe, dip bowl in a larger bowl of hot water. Continue dipping until bombe releases when inverted onto a serving platter. Remove bowl, wipe ice cream drips off platter, and freeze until bombe is very firm, about 2 hours.
Meanwhile, make topping:
Place chocolate and shortening in a heatproof bowl set over boiling water. Stir until chocolate and shortening melt. Let cool slightly (do not cool too much or the chocolate layer will be too thick on top of the bombe). Slowly pour chocolate on top of bombe, spiraling in concentric circles to create a solid center with long drips down the sides. Sprinkle pistachios on top, and freeze until chocolate is set, about 10 minutes.
Run knive under hot water and dry. Use it to slice bombe into wedges, wiping knife between cuts.