In a large bowl, lightly whisk together the flour, baking powder, baking soda, brown sugar, grated nutmeg, cinnamon, ginger and allspice powders. Fold in the chocolate chunks so they’re well coated with flour. Set aside.
In a smaller bowl, whisk together the melted butter, pumpkin puree, yogurt, milk, and vinegar till smooth. Add this to the dry ingredients and fold till just combined. Over mixing will produce dense and chewy muffins.
Spoon the batter into paper lined muffin cups, filling them to 3/4 capacity. Bake them at 180C (350F) for 25 to 30 minutes till they’re done and toothpick pushed into the middle of a muffin comes out clean.
Leave them to cool in the muffin cups for about 15 minutes, then cool them on a rack. These are best eaten the same day. They are good the next day if refrigerated in an airtight container and warmed before eating.