Preheat oven to 350 degrees Fahrenheit and grease well two 9x5 inch loaf pans.

In the bowl of an electric mixer, cream together butter, brown sugar, granulated sugar, and eggs. Add pumpkin puree, vanilla extract, and sour cream and mix until incorporated.

In a separate bowl, stir together flour, baking cocoa, baking soda and salt. Add to wet mixture, along with 1 1/2 c. chocolate chips, and mix just until incorporated, scraping down the sides in between.

Divide batter evenly between the two prepared loaf pans and spread. Sprinkle with 1/2 cup chocolate chips and lightly press them down with your hands. Place them in the oven and set the timer for 40 minutes. Check, by inserting a toothpick into the center, every 3 minutes or so, just until the toothpick comes out clean or with just a few moist crumbs. Don't over bake!

Let them cool in the pans for about 20 minutes before carefully removing them to a cooling rack to cool completely. I like to run a butter knife around the sides to help it release. Let cool, slice and serve.