Pulse Peanut Butter & Cocoa Pebbles in a food processor until finely ground. Or alternatively, place cereal in a large closed plastic bag and smash with a rolling pin or the back of a pan until finely ground.
Mix 5 cups of the ground cereal with 1/4 cup sugar and 5 TBSP melted unsalted butter in a heat proof bowl and firmly press into the bottom and up sides of a 10-inch cast iron skillet. Bake at 350 F for about 8 minutes. Let cool completely on a wire rack before filling.
Meanwhile, toss together sliced bananas, lemon juice, 1 cup sugar, and cornstarch in a large bowl. Add the peanut butter and chocolate chips, toss again, and place into the skillet in an even layer.
Then, make the crumble topping by mixing the remaining 1/3 cup cereal crumbs with 1/2 cup flour, 1/4 cup sugar, and 8 TBSP chilled chopped butter in another bowl, mixing by hand until texture is coarsely clumpy and mixture will hold together in clumps when you squeeze it with your hand.
Spread topping evenly on top of the banana filling with your hand and press it down lightly.
Bake for 25-30 minutes or until the edges are bubbling and the crumble topping is lightly golden brown. Serve slightly warm with vanilla ice cream sprinkled with Peanut Butter & Cocoa Pebbles.