Step 1: Make the Banana Cake
Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
Cream Butter and Sugar: In a large mixing bowl, beat the unsalted butter, vegetable oil, and granulated sugar together until light and fluffy.
Add Bananas and Vanilla: Beat in the mashed bananas and vanilla extract until well combined.
Add Egg Yolks and Buttermilk: Add the egg yolks one at a time, beating well after each addition. Gradually add the buttermilk, mixing until just combined.
Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet mixture, mixing until just combined. Do not overmix.
Bake: Pour the batter into the prepared baking pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely in the pan on a wire rack.
Step 2: Make the Vanilla Wafer Toffee
Melt Butter and Sugar: In a small saucepan over medium heat, melt the unsalted butter. Add the brown sugar, heavy whipping cream, and salt. Stir constantly until the sugar is dissolved and the mixture is smooth, about 2-3 minutes.
Add Vanilla Wafers: Remove from heat and stir in the chopped vanilla wafer cookies until well coated. Let cool slightly.
Step 3: Make the Vanilla Bean Whipped Cream
Beat Cream: In a large mixing bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract together until soft peaks form.
Add Vanilla Bean: Scrape the seeds from the vanilla bean and add them to the whipped cream. Continue beating until stiff peaks form.