Preheat your oven to 350°F (175°C).
Line your muffin tin with parchment paper or muffin liners.
In a large mixing bowl, combine the sour cream, granulated sugar, eggs, vanilla extract, and melted butter. Whisk until smooth.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined.
Fold in the packed brown sugar and additional granulated sugar.
Spoon the batter into the prepared muffin tin, filling each liner about ¾ full.
Bake for 18-20 minutes, or until a toothpick comes out clean.
Let the muffins cool in the tin for about 5 minutes, then transfer to a cooling rack.
