Preheat your oven to 350 degrees Fahrenheit. Grease and flour two 8" round cake pans and set them aside.
To a large bowl add the mashed bananas, the buttermilk, and the eggs, and whisk them together until they're combined. Stir in the canola oil and then the sugar.
Add the flour, baking powder, baking soda and sea salt to a separate bowl and whisk until combined. Add the dry ingredients into the banana mixture and fold them together with a rubber spatula just until no more streaks of flour remain. Do not over-mix or you'll create too many air bubbles in your batter which causes a problem called tunnelling (if you can see long, gaping holes in your baked cake, it means you have over-mixed!).
Divide the batter evenly between the two prepared cake pans and bake at 350 degrees Fahrenheit for about 30-35 minutes or until a toothpick inserted into the middle of one of the cakes comes out clean.
Let the cakes cool in their pans for about 5 minutes and then gently turn them out upside down onto wire racks to cool.
While the cakes are cooling, begin to prepare the frosting by whipping the butter and the peanut butter on high speed until they're well combined and nice and fluffy.
Add the powdered sugar about a cup at a time, mixing well after each addition. Be sure to scrape down the sides of the bowl periodically so that all the powdered sugar gets into the frosting.
Add the whole milk one tablespoon at a time, whipping well after each addition. Continue whipping until the frosting reaches a gorgeous light and fluffy texture that's perfectly spreadable. You may need to add slightly less or slightly more milk depending on what kind of butter and/or peanut butter you use, so it's a good idea to watch closely as you're whipping to ensure you create frosting with a spreadable texture.
Once the cake has cooled, cut each layer in half horizontally using a sharp serrated knife, making sure to turn the cake layer as you cut to create layers as even as possible. If cutting cake layers freaks you out, I guarantee this cake will taste just as amazing as a two-layer cake, so feel free to skip this part altogether!
Frost the cake, making sure to spread a nice thick, even layer of frosting between each of the layers. When making layer cakes I like to add my layers upside down so the flat end is facing upward. This creates a more even top and helps the cake stay straight.
Frost the top and sides of the cake, creating a simple swirl pattern if you wish.
Add the crushed peanuts to the top of the banana layer cake and press them into the frosting just slightly with your palm.