Preheat the oven to 325°F on convection bake. Grease a 9×13 pan with shortening, dust lightly with flour, and set aside.

In a large bowl, whisk the egg whites, buttermilk, sour cream, vanilla, banana extract if using, oil, and mashed bananas until smooth. Sift in the cake mix and gently stir just until combined.

Pour the batter into the prepared pan and bake for 25 to 27 minutes, until the center springs back and a toothpick comes out clean. Cool in the pan on a rack or turn out if frosting the sides.

Beat the butter on medium speed until light and fluffy. Add salt, vanilla, banana extract, and heavy cream. Gradually mix in powdered sugar and whip on high for 2 minutes. Add more cream if needed, tint yellow, then hand-beat to remove air bubbles.

Beat the butter until fluffy. Add salt, vanilla, strawberry emulsion, strawberry powder, and heavy cream. Mix in powdered sugar and whip on high for 2 minutes. Adjust with cream if needed, hand-beat smooth, and transfer to a piping bag fitted with a 1M tip.

Combine chocolate candy melts and heavy cream in a microwave-safe bowl. Heat for 30 seconds, stir, then continue heating in 10-second intervals until smooth. Transfer to a squeeze bottle.

Spread the banana buttercream evenly over the cooled cake. Use a scalloped scraper to create texture, drizzle with ganache, add sprinkles around the edge, pipe strawberry buttercream swirls, and finish each with a maraschino cherry.