To Make the Cake:

1. Preheat oven to 350°F (180°C or gas mark 4). Grease and lightly flour two 9-inch-round cake pans.
2. In a large bowl, cream together sugar and butter with a hand mixer on medium speed, until smooth and light. Add sour cream and eggs, and mix until combined.
3. In a separate bowl, combine baking soda, salt, and flour. Add this dry mixture alternately with the mashed bananas to the butter mixture until combined. Add vanilla and chocolate chips, and fold together until fully incorporated.
4. Divide evenly between the two prepared cake pans. Smooth the surface with the back of a spoon. Bake for 25-30 minutes, until a toothpick inserted in the center of each comes out clean.
5. Remove from oven. Let cool in the pans for 5 minutes. Slide a knife around the edge of the pans for easy removal, and turn them out onto a wire rack. Cool completely before frosting.

To Make the Frosting:

1. Beat the butter and salt in a medium bowl with a hand mixer on medium speed, until combined.
2. Alternate between adding the mashed banana and the confectioners’ sugar to the butter mixture, blending to combine after each addition. Beat in cocoa powder until combined. Continue to beat on medium-high speed until fluffy, about 2 to 3 minutes.
3. To assemble the cake, frost one layer, then top with the second layer of cake, and top that layer with the remaining frosting. Slice and serve!