Set the oven rack to the center position—Preheat to 350ºF (176ºC).
Lightly grease a 9x5-inch loaf pan with cooking spray or vegetable oil. Set aside.
In a large bowl, sift flour, cornstarch, baking soda, baking powder, salt, cinnamon, ginger, nutmeg, cloves, and allspice together. Thoroughly mix with a whisk until combined.
In a medium bowl, whisk together eggs, granulated sugar, and brown sugar until smooth, about 1 minute.
Slowly whisk the vegetable oil into the egg mixture until thoroughly combined, about 30 seconds.
Whisk in pumpkin puree, whole milk, sour cream, and vanilla extract until smooth, about 30 seconds.
Add the wet mixture to the flour mixture. Gently fold the batter until just moistened, making sure to scrape the bottom with some lumps and flour spots remaining. Do not over mix!
Gently stir in the chocolate chips and pecans (if using).
Pour the batter into the greased pan and smooth the top with a spatula. Sprinkle chocolate chips on top.
Bake until a toothpick inserted in the center comes out clean with a few moist crumbs or the internal temperature reaches 195 to 200ºF (90 to 93ºC), about 50 to 60 minutes. Let the bread cool in the pan for 10 minutes.If needed, cover the bread loosely with foil about 45 minutes into baking to prevent the top from getting too brown.
Carefully run a knife or small spatula around the sides of the pan, remove the pumpkin bread and transfer to a cooling rack. Cool completely before slicing.
Using pumpkin pie spice: The cinnamon, ginger, nutmeg, cloves, and allspice can be substituted for 2 ½ teaspoons of store-bought or homemade pumpkin pie spice.
Storing: Store at room temperature in an airtight container for 4 to 5 days.
Freezing: Slices can be individually wrapped in plastic and stored in a bag in the freezer for up to 1 month. Reheat by wrapping in a paper towel and microwave on medium power for 60 to 90 seconds.
Make it Dairy-Free: Use unsweetened cashew or almond milk instead of cow's milk. Use dairy-free sour cream or dairy-free yogurt. Use dairy-free chocolate chips.
Make it Gluten-Free: Use gluten-free flour instead of wheat flour. I recommend Bob's Red Mill 1:1 baking flour.