In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder and salt. Then set aside.
1 ¾ cups (219 g) all-purpose flour, spooned and leveled, ½ cup (40 g) Dutch process cocoa powder, 2 ½ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
In a large bowl, mix together the melted butter, sugar, eggs, milk, sour cream and vanilla.
½ cup (112 g) unsalted butter, melted, 1 cup (200 g) granulated white sugar, 2 eggs, at room temperature, ½ cup (120 ml) whole milk, at room temperature, ½ cup (122 g) sour cream, at room temperature, 2 teaspoon vanilla
Add the dry ingredients to the wet ingredients and mix until combined.
Add in the chocolate chips and fold them into the batter with a rubber spatula.
1 cup (200 g) mini semi-sweet chocolate chips
Cover the muffin batter with plastic wrap and let it rest at room temperature for 30 minutes.
Preheat the oven to 350 degrees. Line 2 muffin pans with 12 liners, lining every other cup so that they are spaced out
After the batter has rested for 30 minutes, scoop the batter into the lined muffin cups. Sprinkle extra mini chocolate chips over each.
½ cup (100 g) mini semi-sweet chocolate chips
Bake the muffins for 20-23 minutes, or until a toothpick comes out from the centers with just a few moist crumbs. (If you choose not to space out the muffins and instead bake them all in one 12 cup pan, keep in mind that the muffins will need about 4-5 extra minutes longer to bake.)
After baking, let the muffins cool in the pan for 10 minutes. Then transfer them to a wire rack to finish cooling.