Make the Cake:

1. Preheat oven to 350°F (175°C). Grease and line two 8-inch round cake pans.
2. In a large bowl, whisk mashed bananas, eggs, sugars, oil, buttermilk, and vanilla until smooth.
3. In a separate bowl, sift flour, cocoa powder, baking soda, and salt.
4. Gradually combine dry ingredients with the wet mixture until just combined.
5. Divide batter evenly into pans and bake for 28–32 minutes or until a toothpick comes out clean.
6. Let cakes cool in the pans for 10 minutes, then transfer to wire racks.

Make the Banana Frosting:

1. In a bowl, beat softened butter with mashed banana and lemon juice until creamy.
2. Slowly add powdered sugar and salt. Beat until light and fluffy. Chill slightly before use.

Make the Chocolate Frosting:

1. In a small saucepan, heat heavy cream until just simmering. Remove from heat.
2. Stir in chocolate chips and butter until melted and smooth.
3. Add vanilla extract. Let cool until spreadable consistency.

Assemble the Cake:

1. Place one cake layer on a serving plate. Spread banana frosting evenly.
2. Top with second layer. Cover top and sides with chocolate frosting.
3. Chill for 15–20 minutes before slicing for cleaner cuts.