Preheat your oven to 350°F (175°C).
Grease and flour two 9-inch round cake pans.
In a large mixing bowl, cream together the butter and sugar until light and fluffy.
Add the mashed bananas and mix until combined.
Beat in the eggs one at a time, followed by the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually add the dry mixture to the wet ingredients, alternating with buttermilk.
Pour the batter into the prepared cake pans.
Bake in the preheated oven for 25-30 minutes or until a toothpick comes out clean.
Allow to cool in the pans for 10 minutes, then transfer to a wire rack.
