Preheat oven to 350℉ (178℃). Grease two 9×5-inch loaf pans with cooking spray, and set aside.
In a small bowl, mix the pumpkin, sour cream, eggs, orange zest, and vanilla extract until smooth, and set aside.
In another medium bowl, blend the dry ingredients: flour, sugar, baking powder, baking soda, cinnamon, nutmeg, allspice, and salt.
Gradually add the wet ingredients to the dry ingredients, and mix until fully combined.
Pour in melted butter and continue stirring until the butter has been fully incorporated into the mixture. Then, divide the batter between the two loaf pans.
If you want to make one loaf with chocolate chips, mix them into half of the batter. Scrape down the sides of the bowl, pour batter into the pan, and spread evenly. Sprinkle a few extra chocolate chips on top.
Bake for 55-60 minutes or until a toothpick inserted in the center comes out clean.
Let the pumpkin bread remain in the pans for about 10 minutes before removing, then turn them out onto a wire rack to cool completely.