Preheat oven to 160°C. Pulse biscuits (150g) in a food processor until broken down into fine crumbs (you should have about 2 cups). Add peanuts (50g) and pulse just to combine. Add butter (85g) and pulse until mixture comes together.
Transfer to a 9"-diameter deep pie dish. Press crumbs firmly onto bottom and up sides of dish. Bake crust until fragrant and edges just start to take on color, 20–25 minutes. Let cool.
Filling
Stir heavy cream (1 cup), butter (2 tbsp), brown sugar (50g), sugar (75g), salt (1/8 tsp), and bittersweet chocolate (28g) in a small saucepan. Whisk until chocolate is melted. Remove from heat and let cool about 10 minutes. Add gelatin (1 envelope) and cocoa powder (75g)to the cooled mixture.
Whisk egg yolks (3) and milk (3/4 cup) in a small bowl to combine, then whisk into sugar mixture. Cook over medium heat, stirring frequently, until mixture begins to thicken and coats the back of a spoon (but do not let it boil), about 5 minutes. Remove from heat and stir iin vanilla extract (1/2 tsp). Transfer to a large bowl and chill until cool, about 10 minutes.
Meanwhile, using an electric mixer on medium-high speed, beat egg whites (3) in a large bowl until soft peaks form. With the motor running, gradually add sugar (1/4 cup) and continue to beat until stiff peaks form, 5–7 minutes.
Mix one-third of egg white mixture into chilled chocolate mixture until smooth. Gently fold remaining egg white mixture into chocolate mixture in 2 additions until incorporated, but don’t overmix.
Pour filling into pie crust; smooth top. Cover and chill overnight.
Topping
Arrange sliced bananas (2) on top of set chocolate filling.
Vigorously whisk cream (3/4 cup) in a large bowl until medium peaks form. Fold in sour cream (1/4 cup) and sugar (2 Tbsp) and salt (1/4 tsp) just to combine. If the cream looks like it could use another whisk or two to hold medium peaks, give it a whisk until it reaches the desired consistency. Using a large spoon, dollop a generous amount of whipped cream in the center of pie. Sprinkle with chopped peanuts.