Preheat oven to 350 degrees and line a cupcake pan with cupcake liners
Using a large bowl, whisk together the flour, cocoa powder, baking soda and salt together
Using a standing mixer, beat together the butter and sugar until light and fluffy
Beat in the egg yolk, milk, and vanilla extract
Gradually mix in the dry ingredients until combined
Spoon about 1 tbsp of the batter into each cupcake liner
Bake in the oven for about 5 - 7 minutes
Set aside to cool