Preheat and Prepare the Pan: Preheat the oven to 425°F (218°C). Generously grease a 6-count jumbo muffin pan with butter or nonstick spray, with nonstick spray recommended, or line the pan with jumbo muffin liners. Set the prepared pan aside.
Combine the Dry Ingredients: In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and optional nutmeg until evenly combined.
Mix the Wet Ingredients: In a separate bowl, whisk the melted and slightly cooled butter, oil, granulated sugar, and eggs until combined. Add the room-temperature sour cream or yogurt, milk, and vanilla extract, then whisk again until incorporated and the mixture is pale yellow. Make the Batter: Pour the wet mixture into the bowl of dry ingredients. Fold everything together with a silicone spatula or wooden spoon until the ingredients are completely combined. If large flour lumps remain, use a whisk to break them up, taking care not to overmix. The finished batter should be thick. Add the Chocolate Chips: Gently fold the semi-sweet chocolate chips into the thick batter until they are distributed throughout.
Fill the Muffin Cups: Divide the batter evenly among the six jumbo muffin cups, filling each one all the way to the top. If desired, sprinkle the tops with coarse sugar for an added crunchy finish.
Begin Baking at High Heat: Place the muffins in the 425°F (218°C) oven and bake for 5 minutes.
Reduce the Oven Temperature: Without removing the muffins from the oven, reduce the temperature to 350°F (177°C) and continue baking for 25–26 minutes.
Check for Doneness: Continue baking until the muffin tops are lightly golden brown and the centers are set. Insert a toothpick into the center of a muffin; the muffins are done when the toothpick comes out clean.
Cool Before Serving: Let the muffins cool in the pan for 10 minutes before serving.
Store Leftovers: Cover leftover muffins and keep them at room temperature for up to 5 days or refrigerate them for up to 1 week. For longer storage, freeze the muffins for up to 3 months and thaw them in the refrigerator or on the counter.