Preheat the Oven: Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt.
Combine Wet Ingredients: In a large bowl, beat together the eggs, sugar, buttermilk, vegetable oil, and vanilla extract until well combined. Stir in the banana puree.
Mix Combine Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix!
Bake: Divide the batter evenly between the prepared pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool completely.
Make the Frosting: In a large bowl, beat the softened butter until light and fluffy. Gradually add powdered sugar and cocoa powder, mixing well. Add heavy cream, vanilla extract, and a pinch of salt, then beat until smooth.
Assemble the Cake: Once the cakes are completely cooled, place one layer on a serving plate. Spread a generous layer of frosting on top, then place the second layer on top. Frost the top and sides of the cake with the remaining frosting.
Garnish: Decorate with chocolate shavings and banana slices.
Serve and Enjoy!: Let the cake sit for about 30 minutes before slicing, allowing the frosting to set up.