Preheat your oven to 350°F (175°C) and grease and line two 9-inch cake pans.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt.
In a large bowl, beat the eggs, granulated sugar, buttermilk, vegetable oil, and vanilla extract until well mixed.
Stir in the mashed bananas into the wet mixture.
Gradually add the dry ingredients to the wet ingredients, mixing until they are combined.
Pour the batter evenly into the prepared pans.
Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
Allow the cakes to cool completely before frosting.
For the frosting, beat the softened butter in a large bowl until light and fluffy.
Gradually add powdered sugar and cocoa powder, mixing well.
Add the heavy cream, vanilla extract, and a pinch of salt, then beat until smooth and creamy.
Place one cake layer on a serving plate and spread a generous layer of chocolate frosting on top.
Add the second cake layer and frost the top and sides evenly.
Garnish with chocolate shavings and banana slices.
Let the cake sit for 30 minutes before slicing for the best results.