Prepare the Crumb Topping: In a medium bowl, whisk together the flour, brown sugar, cocoa powder, cinnamon, and salt. Add the cold cubed butter and work it into the dry ingredients using a pastry cutter, fork, or fingertips until coarse, pea-sized crumbs form. Fold in the mini chocolate chips. Refrigerate the topping while preparing the muffin batter.

Prepare the Oven and Muffin Pans: Preheat the oven to 415°F (213°C). Line or grease regular muffin tins, preparing only every-other well.

Prepare the Pumpkin Muffin Batter: In a large bowl, whisk or blend the oil, maple syrup, mashed banana, eggs, milk, Greek yogurt, pumpkin purée, and vanilla until smooth. In a separate bowl, whisk together the bread flour, buttermilk powder, baking powder, baking soda, cinnamon, nutmeg, and salt. Gently fold the wet ingredients into the dry ingredients until just combined.

Prepare the Chocolate Batter: Transfer approximately one-third of the prepared pumpkin batter into a separate bowl. In a small bowl, dissolve the optional espresso powder in the warm milk, then stir in the cocoa powder. Fold the cocoa mixture into the reserved muffin batter until evenly incorporated. Gently fold in the mini chocolate chips.

Fill and Marble the Muffins: Alternate spoonfuls of pumpkin batter and chocolate batter into each prepared muffin well. Use a toothpick or thin skewer to make one or two gentle figure-eight motions through the batter.

Apply the Crumb Topping: Remove the prepared crumb topping from the refrigerator. Distribute it evenly over the filled muffin wells.

Bake: Place the muffins in the preheated 415°F oven. Bake for 6 minutes at 415°F. Without opening the oven door, reduce the temperature to 350°F. Continue baking for approximately 12–18 additional minutes.

Cool and Serve: Allow the muffins to cool in the pan for approximately 5–10 minutes. Transfer to a wire rack to finish cooling.