Preheat the oven to 350 degrees F. Lightly grease (with cooking spray) and flour a 12-cup muffin tin 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), dark cocoa powder, regular cocoa powder, baking soda, baking powder, and salt. Stir together and 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 (or stand mixer with a whisk attachment) beat the eggs at medium-high speed. Slowly add in the white 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 stream in and beat the oil and then (melted and cooled) butter until completely combined. Beat in the sour cream (or yogurt), buttermilk, and vanilla extract until incorporated.
Add the dry ingredients to the wet ingredients and fold in until JUST combined. You don't want to overmix here or you'll end up with dense muffins.
Spoon the batter evenly into the prepared muffin tin, keeping the scoops mounded. Fill muffin cups a generous 3/4 full.
Bake for 15-21 minutes. (Watch carefully; it can vary depending on actual oven temperature, how full your muffin tin is filled, etc.) As soon as a toothpick comes out 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. Once completely cooled, store the muffins in an airtight container or bag at room temperature. Best used within 2-3 days.
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Notes
Note 1: These muffins have been tested with natural cocoa powder and Dutch-process cocoa powder. If using Dutched cocoa powder, use 1 cup total. While Dutch-process cocoa powder truly makes these the best, regular cocoa powder is delicious as well!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. Add to the recipe in place of regular buttermilk.