Preheat oven to 400 F. Paper-line or grease 14 muffin tin cups.
In a large bowl, mix together pumpkin, sugars, eggs, oil, and buttermilk until well-combined.
Whisk in flour, pumpkin pie spice, baking soda, and salt just until combined. Don't overmix.
Add mini chocolate chips, and stir until just incorporated.
Divide batter evenly among the muffin cups.
Bake for 7 minutes.
Reduce heat to 375F.
Continue baking for 13-18 minutes, or until wooden pick inserted in centers comes out clean. A little melted chocolate is ok, but you don't want wet batter coming out on the toothpick.
Cool in pans for 5-10 minutes, and then remove to wire racks to finish cooling.
Notes
Oil: Neutral oils such as vegetable, olive oil, or canola will work well.
Buttermilk: Regular buttermilk or low-fat buttermilk will work.