Gianduja (a hazelnut-flavored chocolate) and hazelnut praline paste are sold at lepicerie.com
. The first 9 steps can be done in advance.To make banana cream:
Combine bananas, 1/2 cup milk, and lemon juice in a food processor; process until smooth. In a small bowl, sprinkle gelatin over remaining 1/4 cup milk.
Whisk sugar, cornstarch, and salt in a medium saucepan. Whisk in banana mixture, egg yolks, and 1/4 cup heavy cream until smooth and well blended. Bring to a boil over medium heat, stirring constantly with a heatproof rubber spatula. Reduce heat to low and cook 2 minutes, stirring constantly. Stir in butter and reserved gelatin mixture and cook 1 minute, stirring constantly, to dissolve gelatin. Pour mixture into a bowl and set in a larger bowl of ice water; stir occasionally until cool.
Whip remaining 3/4 cup cream just until stiff. Fold into banana mixture. Cover and refrigerate until cold, about 2 hours.To make hazelnut brittle:
Preheat oven to 350°. Spread hazelnuts on a rimmed baking sheet; bake 12 to 15 minutes until toasted. Remove from oven and immediately rub nuts with a clean dish towel to remove as much of skins as possible; let cool. Spray another baking sheet with nonstick spray; set aside.
In a small saucepan, heat sugar over medium heat, swirling pot occasionally, until sugar melts and is amber in color. Stir in hazelnuts to coat with caramel, and pour onto prepared baking sheet. Spread nuts in a single layer with fork; let cool.To make hazelnut crunch:
In a food processor, grind half of hazelnut brittle to a powder.
In a stand mixer with paddle attachment, beat powder, hazelnut paste, confectioners' sugar, and salt until blended. Add rice cereal and beat until blended.To make cake layers:
Adjust 2 oven racks to divide oven into thirds. Preheat oven to 325°. Grease three 10" x 2" round cake pans. Line bottoms with parchment. Grease paper and dust pans with flour; tap out excess. Whisk flour, baking powder, baking soda, and salt in a medium bowl; set aside.
In a stand mixer with paddle attachment, beat butter until creamy. Gradually pour in sugar; beat on medium speed until light and fluffy, about 3 minutes. Add eggs, one at a time, until well blended. Add bananas, buttermilk, and oil; blend until combined. Add flour mixture and beat on low speed until blended. Divide batter between prepared pans and spread evenly. Bake until a toothpick inserted in center of cakes comes out clean, 20 to 25 minutes. Let cakes cool in pans 10 minutes. Run a knife around sides of pans and turn cakes onto racks. Remove parchment paper; let cool.To make hazelnut frosting:
In a small bowl, beat hazelnut paste, butter, confectioners' sugar, and salt until smooth and creamy. Refrigerate until firm enough to spread, about 15 minutes.To make chocolate hazelnut ganache:
In a medium bowl, combine hazelnut paste, chocolate, fudge sauce, and salt. Microwave on medium for 60 seconds, stirring after 30 seconds. Remove and stir until chocolate is melted. Gradually whisk in cream until blended. Cover and refrigerate until cold, about 1 hour.
Right before assembling, set bowl with ganache in a larger bowl of ice water and beat with a handheld mixer until ganache is fluffy and thick enough to spread.To assemble cake:
Place 1 cake layer on a cake stand or serving plate; brush top with whole milk. Spread with half the ganache. Sprinkle evenly with half the hazelnut crunch. Spoon half the banana cream over top and spread evenly. Repeat layering once, then top with last cake layer (brush top with milk), and spread the top with hazelnut frosting. Refrigerate cake at least 1 hour, covered loosely with plastic wrap. To serve, break remaining hazelnut brittle into small pieces and arrange on top of cake.