Preheat oven to 350ºF (175ºC). Grease 9 or 10 muffin cups in your pan. If you want larger muffins, grease 9 cups. Set pan aside.
In a medium bowl, mix flour, cinnamon, ginger, nutmeg, allspice, baking powder, baking soda, and salt together. In a larger bowl, mix pumpkin, milk, honey, oil, eggs, and vanilla. Gradually incorporate dry ingredients to the pumpkin mixture. I like adding my flour in batches: I add about 1/3 of the flour, stir until ingredients are combined, and repeat 2 more times. Fold in chopped chocolate chunks.
Fill 9 muffin cups to the top, or 10 cups 3/4 of the way full.
Wipe down the bowl you used for dry ingredients and add the ingredients for the crumb topping. Using your hands, break up the butter and mix the ingredients together. The mixture should look and feel like wet sand. Sprinkle crumb topping on all the muffin cups.
Bake for 24 to 26 minutes, or until the edges of the muffin tops are slightly browned.
Let the muffins cool in the pan for 5 minutes before taking them out to cool on a plate or rack. Store muffins in an airtight container in room temperature for a few days or freeze them for a few months.