Preheat oven to 350F degrees. Set the oven rack into the middle of the oven.
Line a regular muffin pan with cupcake liners. Set aside.
Add butter and chocolate to a microwave safe bowl and melt in 30-second increments, stirring in between each time. Set aside to cool down and come to room temperature.
Mix cocoa powder, flour, baking soda, baking powder, and salt together until well combined and sift the mixture into a medium size bowl. Set aside.
In the bowl of a stand mixer fitted with the whisk attachment add: eggs, sugar, vanilla bean paste, and sour cream, whisk on medium speed until smooth, for about 1 minute.
Add the cooled butter and chocolate mixture and whisk until smooth, for about 30 seconds.
With the mixer on low speed, slowly add in the flour mixture, about ¼ cup at a time, as soon as the flour has been mixed in, turn off the mixer, to avoid over-mixing the batter.
The batter will be thick and shiny, if needed give it a few stirs with a spatula, to make sure the flour has been well combined.
Fill the cupcake liners ⅔ of the way full with batter.
Bake for 15-18 minutes some ovens may take 20 minutes. At the 15 minutes, mark check the cupcake for doneness. The center of the cupcake should spring back up if gently pressed.