Preheat oven to 350°F. Line 2 muffin pans with parchment liners.
Combine the all purpose flour, Dutch process cocoa powder, hot cocoa mix powder, baking powder, baking soda and salt in a large mixing bowl and whisk to combine. Set aside.
2 cups all purpose flour, ½ cup Dutch-process cocoa powder, ¼ cup hot cocoa mix powder, 1 teaspoon baking powder, 1 teaspoon baking soda, ¼ teaspoon salt
In the bowl of a stand mixer fitted with a paddle attachment (or large mixing bowl), cream the butter and sugar together for 2-3 minutes until light and fluffy scraping the bottom and sides of the bowl as needed.
½ cup salted butter, 1½ cups granulated sugar
Add the eggs, egg yolks and vanilla extract and beat until well combined. Add the sour cream and buttermilk and mix until fully incorporated and the mixture is smooth.
2 large eggs, 2 large egg yolks, 1 tablespoon vanilla extract, ½ cup sour cream, 1 cup buttermilk
Add in the dry ingredients and mix until just combined, scraping the sides and bottom of bowl as needed.
Fill each cupcake liner about ⅔ full of batter and bake for about 17 to 19 minutes or until an inserted toothpick comes out clean.
Let cool in the cupcake tin for about 5 minutes before transferring to a wire cooling rack to cool completely before frosting.