Step 1

Preheat the oven to 350°F (176°C). Line a standard muffin pan with liners or grease the pan  lightly. Set aside.

Step 2

In a medium bowl, whisk together the all-purpose flour, whole wheat flour, espresso powder, salt, baking soda, baking powder, cinnamon, and allspice. Set aside.

Step 3

Place the softened butter and granulated sugar in a stand mixer, fitted with paddle attachment. Beat the butter at low speed for about 30 seconds. Turn the mixer to medium speed and slowly add the granulated sugar, creaming the butter and sugar until it is light in color, about 3 to 4 minutes. Add one egg at a time, and mix until each egg is incorporated fully. Scrape the bowl to ensure that all of the ingredients are incorporated evenly.

Step 4

Add the mashed banana and vanilla, and mix over low speed until just blended (the mixture will look slightly broken). Add the flour mixture in three parts, alternating with two additions of milk. Beat at low speed until just combined. Scrape the bottom of the mixing bowl to ensure that all of the ingredients are incorporated. Stir in the chopped bittersweet chocolate.

Step 5

Using a spoon or batter scoop, divide the batter evenly among the muffin pan–this recipe will make 11 muffins. Bake for 23 to 26 minutes, or until the muffins are springy to touch and a toothpick inserted in the center comes out clean. Place the muffin tin on cooling rack, and allow the muffins to cool in the pan for a few minutes. Remove the muffins from pan, place on a cooling rack, and eat warm or allow to cool to room temperature.

Step 6

Muffins best served the day of baking, but can be stored in an airtight container or Ziploc bag for an additional 2 to 3 days. Place a paper towel over the surface of the muffins to prevent gumminess on the surface during storage.