In another medium sized bowl, mash the bananas with a fork until they are completely mashed.
4 medium-sized ripened bananas
In a stand mixer fitted with the paddle attachment, beat the butter, oil, and sugar on medium-high speed until light and fluffy, about 5 minutes. With a rubber spatula, scrape down the sides of the bowl.
1 cup unsalted butter, 1 Tablespoon vegetable oil, 1 ½ cups granulated sugar
With the speed turned down to medium, add the eggs, one at a time, mixing until each is fully incorporated before adding the next (about 30 seconds for each addition). Add the vanilla and banana extract and mix on low.
4 large eggs, 1 teaspoon vanilla extract, ½ teaspoon banana extract
With the mixer on low, add the dry ingredients, alternating with the buttermilk into the creamed butter mixture, starting and ending with the dry ingredients. I do it in the three parts. Mix until just incorporated and remove bowl from stand mixer.
1 ¼ cup buttermilk
With a rubber spatula, fold in the mashed bananas until fully incorporated. Make sure to scrape down the bottom of the bowl.
Divide the batter evenly between the three pans, using a kitchen scale for the best accuracy. I had about 22 ounces in each one.
Carefully smooth the batter with a small offset spatula and bake until a toothpick comes out clean, or with only a few crumbs on it, about 33-36 minutes.
Let pans cool on wire rack for 10 minutes, then transfer the cakes out of the cake pans and onto cooling racks. Let them cool completely.
For the cookie butter filling
In a stand mixer fitted with a paddle attachment, beat the butter and Biscoff spread, cream, powdered sugar, and salt on medium speed until light and fluffy, about 3 minutes.
1 cup unsalted butter at room temperature, 2 ½ cups Biscoff spread or other cookie butter, 3 cups powdered sugar, pinch of salt
With a rubber spatula, beat by hand to push out all of the air pockets.
For the Biscoff buttercream
In the bowl of a stand mixer, fitted with the paddle attachment, beat the butter and Biscoff crumbs on medium speed until smooth (this will take about one minute).
Scrape down the bowl and add the powdered sugar. Turn your mixer on the lowest speed. While it’s mixing, slowly add the vanilla, salt and heavy whipping cream. Beat on medium high speed for 4-5 minutes. Scrape down the bowl as needed.
Beat frosting by hand with a wooden spoon to get the air bubbles out and create a smooth buttercream.
Assembly
Smear a dollop of frosting in the middle of the cake board to act as a glue. Place the first cake layer on a cake board, bottom side down. Gently and evenly spread half of the cookie butter filling, followed by a some melted cookie butter (the melted cookie butter is optional).
Place the second cake layer bottom side up, right on top of the cookie butter filling.
Place your final cake layer, bottom side up, right on top of the second layer of filling.
Apply a thin layer of the Biscoff buttercream around the entire cake and freeze the cake for 10 to 15 minutes.
Once the crumb coat is set, finish frosting and decorating the cake with half of the Biscoff buttercream and half of the cookie butter filling. Garnish with Biscoff cookie crumbs.