For the Cake
Preheat your oven to 275 F (135 C). Grease an 8 X 11 inch (20.3 x 28–cm) baking dish and line with parchment paper allowing excess to hang over the sides...this will help you take the cake out of the pan later.
In an electric stand mixer fitted with the paddle attachment add the butter, granulated sugar and brown sugar and mix on medium high for about 4-5 minutes or until well blended and fluffy. Add in the eggs, one at a time, taking care that the first is well blended before adding in the second. Scrape down the sides and bottom of the bowl to make sure everything is well blended. Add in the mashed bananas, espresso and vanilla and run the machine for 1 minute more or until everything is thoroughly combined.
In a small bowl whisk together the all-purpose flour, cake flour, baking soda, sea salt, cinnamon, and nutmeg. Alternate adding the flour mixture and the buttermilk to the banana mixture in two batches, starting with the flour mixture and ending with the buttermilk. Run the machine on low until almost combined. Add the chocolate and run the machine on low for 1 minute more.
Pour the batter into your prepared pan, smoothing the top with the back of a spatula, and bake in the center of the oven for 90 minutes or until a toothpick comes out clean. Some bits of chocolate on the toothpick are fine. Let cool for 40 minutes before removing from the pan and placing on a rack to finish cooling.
For the Buttercream
While the cake cools, make the buttercream by adding the butter and confectioners sugar to an electric stand mixer fitted with the paddle attachment. Run the machine on low until well blended and there are no lumps. You may need to put a towel over the machine so that powdered sugar doesn't fly everywhere. Add in the milk, vanilla and finely-ground espresso and run the machine for 2 minutes more or until well-blended and creamy, dreamy looking. Add more milk if necessary in small, 1 teaspoon increments to get to the desired consistency. Smear the buttercream over the top, leaving a gutter around the edge so that you can see the cake. Sprinkle with cacao nibs if using and serve. Try to only eat one slice. I dare you.
Thank you for making this cake! Please tag me @displacedhousewife #displacedhousewife so I can see your beautiful creations! xo