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1
Preheat oven to 350 degrees F. Grease a muffin tin with cooking spray, wiping off any excess on the top of the tin.
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2
In a bowl, whisk together the flour, cocoa, baking powder, baking soda, and salt.
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3
In the bowl of a stand mixer fitted with the whisk attachment, begin beating the eggs and yolk on medium-high speed. Slowly stream in the sugar while beating and continue to beat until mixture is pale and thickened and ribbons down from the beater when lifted. Scrape down sides of the bowl and beater as needed.
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4
Beat in the oil until fully incorporated. Scrape down bowl and beater and beat in sour cream and vanilla until evenly incorporated.
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5
Add the dry ingredients to the wet and fold in until only a few visible streaks remain. Add 8 1/2 ounces of the chocolate chips and fold in just until all ingredients are evenly incorporated.
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6
Spoon the batter into prepared muffin cups, smoothing the tops if needed but keeping the scoops mounded. Top each with some of the remaining chocolate and sprinkle with the coarse sugar.
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7
Bake muffins in preheated oven until a toothpick inserted into the center of one comes out free of wet batter (it may come out with melted chocolate), 18 to 20 minutes. Cool muffins in pan for about 15 minutes before transferring to a wire rack to cool completely.
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8
Store muffins in an airtight container at room temperature, or in the refrigerator to keep for more than a few days.
