Preheat the oven to 425 degrees Fahrenheit. Line a muffin pan with muffin liners and spray lightly with nonstick cooking spray. Set aside.
Add ¾ cup Dutch process cocoa powder, 1 ½ cups all-purpose flour, 1 ½ teaspoons baking powder, ¾ teaspoon baking soda, and ½ teaspoon salt to a medium bowl and whisk until combined. Set aside.
Add ½ cup packed dark brown sugar, ⅓ cup granulated sugar, and ½ cup vegetable oil to a large bowl and mix until combined.
Add ½ cup buttermilk, ½ cup sour cream, and 1 teaspoon vanilla extract and mix until combined. Add 2 large eggs and mix until combined.
Add the flour mixture to the sugar mixture and mix until fully combined. Fold in ¾ cup roughly chopped semi-sweet chocolate by hand.
Scoop the batter evenly into the prepared muffin pan, filling the wells about ⅔ of the way full. You can add additional chopped chocolate on top if you would like.
Bake for 6 minutes at 425 degrees Fahrenheit before lowering the temperature to 375 degrees Fahrenheit and baking for an additional 11-13 minutes.
Remove the pan from the oven and let the muffins cool for 5 minutes or so in the pan before removing them to a baking rack to finish cooling completely.
Notes
Storage Instructions
Room Temperature: Store leftover muffins in an airtight container at room temperature for up to 5 days.
In the Refrigerator: For longer storage, store in the fridge for up to 8 days. You can reheat them in the microwave for 10-15 seconds for a warm muffin!
In the Freezer: Freeze in an airtight container or freezer bag for up to 3 months. Let the muffins thaw to room temperature by setting them on the counter for a few hours.