In a large bowl, whisk together the granulated sugar, all-purpose flour, cocoa powder, baking powder, baking soda, salt, and dark chocolate chips until well combined.
1/2 cup granulated sugar, 1 cup + 1 tablespoon all-purpose flour (127 grams), 1/4 cup cocoa powder (25 grams), 1/2 tablespoon baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon salt, 1/2 cup + 2 tablespoons dark chocolate chips (108 grams)
In a separate bowl, whisk together the whole milk, canola oil, sour cream, egg, and vanilla extract until smooth.
1/2 cup whole milk,, 1/4 cup canola oil, 1/4 cup full-fat sour cream,, 1 large egg,, 1 teaspoon vanilla extract
Pour the wet ingredients into the dry ingredients. Using a rubber spatula, gently fold the batter until just combined but DO NOT overmix.
Cover the bowl with a kitchen towel and let the batter rest for 30 minutes. Preheat your oven to 425°F (218°C) and line a 6-cup muffin tin with muffin liners.
Evenly divide the batter among the six muffin liners and top with extra chocolate chips. Bake at 425°F for 5 minutes, then without opening the oven door, reduce the temperature to 350°F (177°C) and bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean. Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack.