Preheat oven to 350°. Line 8 jumbo muffin cups with paper liners, or coat well with cooking spray.

In the top of a double boiler, melt chocolate with espresso powder. Remove from heat, and let cool. In a large bowl, whisk together flour, sugar, baking soda, salt, cinnamon, and nutmeg. Make a well in the center of mixture. In a medium bowl, whisk together mashed banana, eggs, oil, buttermilk, molasses, and vanilla. Add to dry ingredients, stirring just until moistened. Spoon 1½ cups (330 grams) batter into a separate bowl, and stir in chocolate-espresso mixture.

Spoon banana batter into prepared muffin cups, filling two-thirds full. Using the back of a spoon, make a shallow trench in the middle of each muffin cup. Spoon in chocolate-espresso batter, and swirl with a wooden pick. Bake until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Let cool in pans for 5 minutes. Serve warm or at room temperature.