Preheat oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or line with cupcake liners. Set aside.

Coarsely chop the chocolate. Place in a medium heat-proof bowl along with the butter. Microwave on high in 10 second increments, whisking after each until completely smooth and chocolate has melted. Cool for 5 minutes, then whisk in the sugar, eggs, vanilla, and sour cream. Set aside.

Whisk the flour, cocoa powder, baking soda, and salt together in a large bowl. Add the wet ingredients and fold to completely combine—make sure there are no flour spots at the bottom of the bowl. Fold in the chocolate chips. Batter will be thick.

Spoon the batter evenly into each cup or liner, filling each all the way to the top. Bake the muffins for 5 minutes at 425°F, then, keeping the muffins in the oven, reduce the oven temperature to 350°F (177°C). Bake for an additional 15–17 minutes or until a toothpick inserted in the center comes out clean. The total time these muffins take in the oven is about 20–22 minutes, give or take. Allow the muffins to cool for 5 minutes in the muffin pan, then transfer to a wire rack to continue cooling.

Muffins stay fresh stored at room temperature for a few days, or in the refrigerator for up to 1 week.