Spread vanilla ice cream in an even layer on bottom and up sides of a 2 1/2-quart heatproof bowl with a 9 1/2 inch diameter rim. Ice cream should form a concave layer that comes up to the rim of the bowl. Freeze until set, about 1 hour ( may need to keep pushing ice cream up sides if ice cream falls as it is trying to re-freeze).
Meanwhile, 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 1 minute. Whisk until combined. Refrigerate, stirring occasionally, until thickened and spreadable, about 1 hour.
Spread chocolate-banana mixture in a thin even layer over entire surface of vanilla ice cream layer. Freeze until firm, about 30 minutes.
Spread pistachio ice cream in a flat, smooth layer over chocolate-banana mixture, but no up sides. Freeze until firm, about 30 minutes.
Spread chocolate ice cream in a flat, smooth layer over pistachio ice cream. Freeze until firm, about 30 minutes.
Slice banana bread lengthwise into 1-inch slices. Cut pieces to cover top of ice cream bombe (square and triangle shapes work best).
Cover with plastic wrap and freeze overnight or up to 3 days.
To unmold the bombe, dip bowl in a larger bowl with very hot water or use a small kitchen torch to warm outside of bowl. Continue dipping or torching bowl until bombe releases when inverted onto a serving platter. Remove bowl, wipe drips off platter, and freeze until firm, about 1 hour.
Squeeze magic shell on top, slowly, to create a solid center with long drips down the sides.
Sprinkle pistachios on top, and freeze until chocolate is set, about 5 minutes.
Run a large knife under hot water, and dry well. use it to slice the bombe into wedges, as you would a cake, wiping knife between cuts.