Preheat oven to 350 degrees F. Line a standard 12-cup muffin tin with paper liners.
In a large bowl, whisk together flour, cocoa powder, baking powder, baking soda and salt.
In a separate bowl, whisk together melted butter, granulated sugar, brown sugar and vanilla extract. Add eggs, whisk to combine. Add sour cream, whisk until smooth.
Add the wet ingredients to the dry ingredients. Using a wooden spoon or spatula, combine just until incorporated – do not over-mix. Fold in the chocolate chips.
Divide batter evenly among the prepared muffin cups, filling each cup almost to the top. Sprinkle additional chocolate chips and coarse sugar on top.
Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for a few minutes, then transfer muffins to a wire rack to cool completely.