Preheat oven to 375º F. Grease and dust 10 cup Bundt pan with extra cocoa powder.
In a bowl mix banana with lemon juice. Set aside.
In another bowl, mix flour, cocoa, baking soda and salt. Set aside.
In a large bowl beat oil and sugar until light and fluffy. Beat in eggs, one at a time. Add vanilla.
Blend in flour and buttermilk in two additions. Fold in bananas and chocolate chips.
Bake 55 minutes. Cool in pan for 30 minutes, then turn out onto cake a cake plate.
To make ganache: place chocolate chips and milk in a microwave dish. Cook on high for 1 minute. Whisk until smooth. Pour over cake. Drizzle with caramel sauce.