Preheat oven to 425°F. Break chocolate bar into pieces and melt in 30-second intervals in the microwave, stirring between each interval, until smooth; set aside.
In a large bowl combine flour, cocoa powder, baking powder, baking soda, espresso powder, and salt.
Combine oil, granulated sugar, brown sugar, and melted chocolate in a bowl, whisking until smooth.
Whisk in eggs, one at a time, until smooth. Whisk in sour cream, heavy cream, and pumpkin.
Create a well in the center of the dry ingredients; pour wet ingredients into the dry. Use a rubber spatula or wooden spoon to stir until the flour has just combined (a few streaks of flour may remain - this is OK. It's better to not over mix the batter).
Fold in the chocolate chips (not the miniature ones) until just combined. Line a 12-cup muffin tin with cupcake liners. Fill each muffin cup 3/4-full with batter. Sprinkle the tops of each muffin with miniature chocolate chips, if desired.
Bake at 425°F for 5 minutes, lower the temperature to 375°F and bake an additional 14 to 16 minutes or until a toothpick inserted into the center of the muffin comes out clean.
Cool muffins completely in pan before transferring to a wire rack to cool completely.