Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Grease and flour two 9-inch round cake pans.

In a large bowl, cream together the softened butter, sugar, eggs, and vanilla extract using an electric mixer on high speed for 1.5 minutes.

Add the buttermilk and mix until combined.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the flour mixture to the wet ingredients, alternating with the mashed banana.

Beat until just combined, about 2 minutes. Do not overmix.

Pour the batter evenly into the prepared cake pans.

Bake in the preheated oven for 30 to 35 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

To prepare the frosting, mix together the powdered sugar, cocoa, and salt in a large bowl.

Gradually add the softened butter, beating with an electric mixer, and slowly add milk until the frosting reaches a smooth and spreadable consistency.

Beat on high speed for 3 to 4 minutes, adding small amounts of milk if needed to achieve the desired consistency.

Once the cakes are completely cooled, fill and frost the layers with the chocolate frosting.

Slice and serve.