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.
¼ cup (56 g) unsalted butter, softened, ¾ cup (150 g) granulated white sugar
Then add in the egg, egg yolk and vanilla and mix until pale in color and smooth, about 1 minute.
1 egg, at room temperature, 1 egg yolk, at room temperature, 1 teaspoon vanilla
Next mix in the milk, sour cream and pumpkin puree.
¼ cup (60 ml) whole milk, at room temperature, ¼ cup (62 g) sour cream, at room temperature, ½ cup (122 g) canned 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.
1 cup (125 g) all purpose flour, sifted, ¼ cup + 2 tablespoon (30 g) black cocoa powder, sifted, 2 ½ teaspoon pumpkin pie spice, ½ teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon 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.