In the bowl of a stand mixer or with an electric hand mixer, combine the partially melted butter and brown sugar until thoroughly combined.
Add the egg, pumpkin puree and vanilla extract. Mix until combined.
Add the flour, cornstarch, cinnamon, soda and salt to the bowl. Start mixing slowly.
While the dry ingredients are being incorporated, add in the buttermilk. Mix until completely incorporated.
Fold in the semi-sweet chocolate chips.
Cover the bowl and let the batter rest while the oven preheats.
Preheat the oven to 425° F.
While the oven is preheating, add a muffin liner to every other space in your muffin pan. This will allow the muffins to be large and bakery style.
Scoop the batter into the muffin liners, overfilling the liners purposefully. (See photo above).
Bake at 425° F for 7 minutes and then reduce the heat to 350° F for an additional 10 minutes or until a toothpick comes out clean. (This will encourage the muffins to grow tall and have a large bakery style muffin top).
Allow the muffins to cool on a cooling rack and enjoy!
Store in an air tight container for up to 4 days.