Prepare your preferred banana cake recipe and allow it to cool thoroughly before continuing. Begin crafting the caramel sauce as the cake cools.

In a medium saucepan, combine granulated sugar, water, and fine sea salt. Set over medium-high heat, whisking continuously until the mixture reaches a boil. Cease whisking and permit it to boil undisturbed, swirling the pan now and then until a deep amber hue is achieved.

Carefully incorporate the butter, whisking until fully blended. Remove from heat and gradually add the heavy cream—be cautious, as it will steam. Stir until completely smooth.

Allow the caramel to cool slightly, then pour into a jar or heatproof bowl. Chill until set.

To make the frosting, beat the softened butter until pale and airy. Blend in powdered sugar, vanilla, half a cup of the chilled caramel sauce, and a pinch of sea salt. Continue beating until light and creamy. Chill slightly for a firmer spread, if needed.

To assemble, spread half of the frosting evenly over the base cake layer. Spoon on some caramel sauce and distribute it smoothly. Lay the second cake layer on top and cover with the remaining frosting.

Prior to serving, garnish with diced bananas, a scattering of chopped nuts, and chocolate shavings as desired. Drizzle the remaining caramel sauce over the top and finish with a sprinkle of coarse sea salt.