Preheat oven to 425 degrees. Spray a regular-sized muffin pan with nonstick spray or line with cupcake liners.
In a large bowl, whisk the flour, cocoa powder, baking soda, and salt together in a large bowl. Set aside
Using a sharp knife, coarsely chop the chocolate and place in a medium microwavable glass bowl along with the butter.
Place bowl in microwave. Melt butter and chocolate on high in 10 second increments, whisking after each until completely smooth.
Remove bowl from microwave, and then whisk in the sugar, eggs, vanilla, and sour cream.
Add the wet ingredients to the dry ingredients, and fold to completely combine. Be sure to scrap the bottom of the bowl to get everything mixed.
Fold in the peanut butter chips. The muffin batter will be on the thick side.
Spoon the batter evenly into each prepared muffin cup, filling each almost to the top.
Bake the muffins for 5 minutes at 425 degrees then, while keeping the muffins in the oven reduce the oven temperature to 350 degrees.
Bake for an additional 15-17 minutes or until a toothpick inserted in the center comes out clean.
Remove muffins from oven and allow to cool for 5 minutes in the pan, then transfer to a wire rack to continue cooling.