Add the milk and cream to a medium saucepan over medium heat. Bring to a simmer, stirring often.

In a medium bowl, whisk together the eggs, sugar, and cornstarch until well blended.

Spoon a little of the hot milk over the egg mixture and whisk well. Repeat with another few spoonfuls of the hot milk, to temper the eggs.

Pour the egg mixture into the saucepan and continue to cook over medium low heat for about 10 minutes or until it thickens and coats the back of a spoon.

Once thickened, remove from the heat and stir in the butter, vanilla extract, banana extract, and salt.

Pour the pudding through a sieve or fine mesh strainer into a container, to remove any lumps.

Place a piece of plastic wrap or wax paper directly on top to prevent a skin from forming. Refrigerate for at least 3 hours, or until completely chilled and set.

Preheat the oven to 325°F. Liberally spray 3 (8-inch) round cake pans with non-stick baking spray or grease and flour.

In your mixer bowl, add oil and butter and beat for 2 minutes on high speed. Slowly add in sugar and beat on high speed for an additional 4-5 minutes until very pale yellow and fluffy.

Next, add eggs, one at a time, combining well after each addition and scraping down the sides as needed.

Turn your mixer down to its lowest speed, slowly add flour into batter in two increments, then add baking powder, salt, and baking soda.

Lastly, add sour cream, milk, vanilla extract, and banana extract, mix until just combined.

Evenly pour cake batter into prepared baking pans and place in the oven to bake for 15-25 minutes or until a toothpick inserted into the center comes just barely clean.

Remove cakes from the oven and rest in pans for 10-15 minutes. Invert cakes from pans onto cooling racks, then allow to cool to room temperature.

Whip cream, sugar, vanilla, and cream of tartar if using until stiff peaks.

Chill until ready to use.

Place one layer of cake on a cake stand or serving plate. Spoon ½ of the banana pudding onto the cake and use a spatula to spread it to the edge.

Sprinkle crushed nilla wafers on top.

Place the next layer of cake on top, then repeat the previous step.

Add the last layer of cake on top, then proceed to frosting the cake with the premade whipped cream.

Decorate the top of the cake with Nilla wafers and slices of banana if desired.