Preheat 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.
To a large bowl add the softened, unsalted butter and granulated sugar. Cream together with an electric mixer until fluffy, 1-2 minutes.
Then add in the egg, egg yolk and vanilla. Mix until pale in color and smooth, 1-2 minutes.
Alternate adding the dry ingredients and the buttermilk to the eggs/butter/sugar mixture until all is just combined and the batter is smooth. (Scrape down the sides of the bowl as necessary.)
Lastly fold in the red gel food coloring just until combined.
Divide the batter among the liners. Each will be about ¾ of the way full. Bake the cupcakes for 18-21 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.