Preheat oven to 375°F and line a muffin tin with liners or spray with cooking oil. (I recommend using liners for easy removal and less mess).
In a stand mixer with a whisk attachment, add in eggs and egg yolk and beat on medium speed for approximately 2-3 minutes.
2 eggs, 1 egg yolk
Add in sugar and continue mixing until the mixture has thickened and turned pale (View video for visuals).
1 1/2 cups white sugar
Add in oil and keep on mixing.
1/2 cup oil
Once well combined, add in sour cream and vanilla and mix.
1 1/2 cups sour cream, 1 tbsp vanilla
Add in all of the dry ingredients and slowly mix until just combined. A soft batter will form. Do not overmix!!
2 cups flour, 3/4 cups unsweetened cocoa powder, 3 tsps baking powder, 1 tsp baking soda
Toss in chocolate chips and mix for a few seconds.
1 cup semi-sweet chocolate chips
Evenly divide between 24 muffin liners. The batter will go to nearly the top of the muffin liners. Sprinkle with additional white sugar.
Bake at 375°F for 10 minutes, then reduce temperature to 350°F and continue baking for an additional 5-10 minutes.
Allow to slightly cool in muffin pan. Serve warm & enjoy!