Preheat oven to 425 degrees F. Line a standard muffin tin with paper muffin cups or grease with a non-stick cooking spray.
In a small bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a large bowl, melt together the chocolate butter until completely smooth. (You can do this in a glass bowl using a microwave in 30-second bursts, stirring after each; or you can use a heatproof bowl set over a small saucepan of barely simmering water.)
Whisk the sugar into the melted chocolate mixture until combined, then whisk in the sour cream and vanilla extract. Finally, gently whisk in the eggs until incorporated, taking care not to overmix at this stage.
Add the dry ingredients to the wet ingredients and use a rubber spatula to gently fold and mix until just a little bit of dry flour remains. Add the chocolate chips and gently mix them together until they are evenly distributed and no dry flour remains.
Divide the batter evenly between the muffin cups (I find a ¼ cup scoop to work perfectly).
Bake for 5 minutes, then reduce the oven temperature to 350 degrees F and continue baking for 15 to 20 minutes, until a toothpick inserted into the center comes out with moist crumbs attached. Allow to cool for 5 minutes, then remove the muffins from the pan to a wire cooling rack until cooled completely.
Store in an airtight container at room temperature for up to 4 days, or wrap individually in plastic wrap and place in an airtight container or resealable bag and freeze for up to 3 months.