Preheat oven to 350 degrees and line a standard cupcake pan with 12 cupcake liners.
Start by creaming together the butter and sugar in a large bowl with an electric mixer for 1-2 minutes.
Then add in the egg, egg yolk and vanilla and mix until pale in color and smooth, about 1 minute.
Next mix in the milk, sour cream and pumpkin puree.
In a small bowl whisk together the sifted purpose flour, sifted black cocoa powder, pumpkin pie spice, baking powder, baking soda, and salt.
Add the dry ingredients to the wet ingredients and mix until the batter is smooth.
Evenly divide the batter among the cupcake liners. They should be about ¾ way full.
Bake the cupcakes for 17-19 minutes, or until a toothpick/cake tester comes out clean from the centers.
Let the cupcakes cool in the pan for 5 minutes then transfer to a cooling rack to cool completely.