Preheat oven to 350°F. Lightly grease with cooking spray and flour a 12-cup muffin pan and set aside.
In a large bowl, sift together the flour (make sure to spoon and level your measurement so you don’t pack in too much flour), both kinds of cocoa powder, baking soda, baking powder, and salt. Stir together, then add in the mini and regular chocolate chips. Stir to coat the chips in the dry ingredients. This keeps them from sinking to the bottom.
In another large bowl, use a hand mixer to beat the eggs at medium-high speed. You can also use a stand mixer with a whisk attachment. Slowly add in granulated sugar and then brown sugar while continuing to beat. Beat until the mixture is pale and thickened, scraping the sides of the bowl as needed. Slowly pour in oil while beating, then melted and cooled butter until completely combined. Beat in sour cream, buttermilk, and vanilla extract until incorporated.
Add the dry ingredients to the wet ingredients and fold until just combined. You don’t want to overmix or you’ll end up with dense muffins.
Spoon the batter evenly into the prepared muffin tin. Fill muffin cups 3/4 full.
Bake for 15–21 minutes and be sure to watch carefully. As soon as a toothpick comes out clean or with moist crumbs, they are done baking.
Remove and allow to cool in the pan for 5–8 minutes before removing to a cooling rack to finish cooling.
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Notes
Note 1: If you don't have regular cocoa powder, just use 1 full cup Dutch-process cocoa powder.Note 2:To make your own buttermilk, pour 3/4 cup of milk into a bowl, stir in 2 teaspoons of white vinegar or fresh lemon juice, and let sit for about 5 minutes. Storage: Once completely cooled, store the muffins in an airtight container or bag at room temperature. Best eaten within 2–3 days.