Preheat the oven to 350 degrees. Line a cupcake pan with 12 liners and set aside.
In a small bowl whisk together the flour, cocoa powder, baking powder, baking soda and salt. Then set aside.
1 cup + 2 tablespoon (141 g) all-purpose flour, spooned and leveled, 1 ½ tablespoon (8 g) cocoa powder, Dutch process, ¾ teaspoon baking powder, ⅛ teaspoon baking soda, ¼ teaspoon salt
To a large bowl add the softened, unsalted butter and granulated sugar. Cream together with an electric mixer in high speed until fluffy, 1-2 minutes.
5 tablespoon (70 g) unsalted butter, softened, ¾ cup (150 g) granulated white sugar
Then add in the egg, egg yolk and vanilla. Mix on medium speed until pale in color and smooth, 1-2 minutes.
1 egg, at room temperature, 1 egg yolk, at room temperature, 1 teaspoon vanilla
Alternate adding the dry ingredients and the buttermilk to the eggs/butter/sugar mixture on low speed until all is just combined and the batter is smooth. (Scrape down the sides of the bowl as necessary.)
½ cup (120 ml) buttermilk, at room temperature
Lastly fold in the red gel food coloring just until combined.
1 ½ teaspoon red gel food coloring
Divide the batter among the liners. Each will be about ¾ of the way full. Bake the cupcakes for 17-20 minutes, or until a toothpick comes out clean from the centers.
When the cupcakes are done baking let them cool in the pan for a 10 minutes, then transfer them to cooling rack to completely cool.