Prep. Preheat oven to 375º F. Prepare muffin tin with liners or by spraying with nonstick baking spray. Sift together flour, cocoa powder, baking powder, baking soda, and salt into a large bowl. Set aside.
Mix. Mix together brown sugar,eggs, pumpkin puree, vanilla extract, and buttermilk, being sure to incorporate well. Add flour mixture about 1/2 cup at a time, stirring just to combine. Fold in chocolate chips.
Bake and serve. Spoon muffin batter into prepared muffins tins, filling about ¾ full. Place in preheated oven and bake until the lightly spring back to the touch and a toothpick comes out clean when inserted in the center, about 12 minutes for mini-muffins and 25-30 minutes for full-size muffins. Allow to cool slightly before serving.