Preheat oven to 350 degrees.
Grease bottoms and sides of two 9-inch round, high-sided baking pans. Place parchment paper rounds into bottom of pans then grease the parchment paper. Set aside.
In a large mixing bowl, sift together dry ingredients: flour, cocoa powder, baking soda, baking powder, salt and espresso powder. Set aside.
In a separate bowl, use an electric mixer to beat butter and sugar until creamy smooth – approximately 3 minutes.
Add eggs, one at a time, beating after each addition until incorporated. Scrape down the sides of the bowl with a spatula as needed.
Beat in mashed bananas, sour cream and vanilla.
Reduce mixer speed to low. Alternate between adding 1/4 cup of buttermilk and half of flour mixture in 2 separate batches, beating after each addition until just barely combined. Avoid overmixing.
Coat mini chocolate chips with 1 tablespoon of flour then gently stir them into the cake batter with a spoon until evenly distributed.
Evenly divide batter between the 2 baking pans, smooth the tops with a spatula then bake for 40 to 45 minutes or until a tester inserted into the center of cake comes out clean.
Remove baking pans from oven then place on a wire rack to cool completely, approximately 2 hours, before frosting.