Preheat the oven to 350°F. Butter a 9-inch square or round cake tin.
In a medium bowl, mix together the flour, cocoa powder, baking soda, cinnamon and salt.
In a large bowl cream together the butter and sugars with an electric mixer. Beat in the eggs one at a time. Mix in the vanilla, then beat in the mashed banana.
Turn the flour mixture into the banana mixture in two batches and stir until just blended. Mix in the chocolate chips.
Scrape the batter into the pan, smooth the top and bake for about 35 minutes, until a wooden skewer either comes out clean or with a little bit of melted chocolate from the chips. The top of the cake should feel springy but firm.
Cool in the pan on a wire rack for 10 minutes, then turn the cake out of the pan and finish cooling, upright, on the wire rack.
Notes
This was also the first time I realized a cake could be a breakfast food, a snack food, an anytime of day cake. This was a very good discovery. In Dinner Solved there is a Berry Coffee Cake which is another any-time-of-day cake. A dollop of whipped cream or a sprinkle of powdered sugar can also turn an anytime of day cake into a dessert. Try whipped chocolate ganache for another topping option!