Preheat your oven to 375°F.
Prepare your muffin tins with muffin cups.
Combine the flour, ¼ Cup of cocoa powder, baking powder, baking soda, and salt in a medium bowl, whisking to mix.
In another bowl, combine the coffee and remaining cocoa powder, mixing until smooth.
In your mixer bowl, beat the sugar, oil, eggs and egg yolk, and vanilla, until fully combined.
Add in the mixture of coffee and cocoa powder and combine, careful not to over-mix.
Add half of the dry mixture and combine. Then, add the sour cream and mix. Finally, add the rest of the dry mix and combine. If it's a little under-mixed at each step, that's fine.
Add in 2 cups of chocolate chips and fold to mix, but avoid over-mixing.
Spread your batter in muffin cups, filling about ¾ of the way. You should be able to make 20-24 muffins.
Sprinkle the remaining chocolate chips across the top of the batter.
Bake for about 18 minutes, or until a toothpick comes out clean. Let cool slightly and enjoy!