Prepare the Cupcakes:
Preheat the oven to 350°F (175°C). Line a standard cupcake pan with 12 cupcake liners. In a large bowl, cream together the butter and sugar using an electric mixer until light and fluffy (about 1-2 minutes). Add the egg, egg yolk, and vanilla extract, mixing until smooth.

Add Wet Ingredients:
Mix in the milk, sour cream, and pumpkin puree until fully combined.

Combine Dry Ingredients:
In a separate bowl, whisk together the sifted flour, black cocoa powder, pumpkin pie spice, baking powder, baking soda, and salt.

Mix Wet and Dry Ingredients:
Gradually add the dry ingredients to the wet ingredients, mixing until smooth. Be careful not to overmix.

Bake the Cupcakes:
Divide the batter evenly among the cupcake liners, filling them about ¾ full. Bake for 17-19 minutes or until a toothpick comes out clean. Let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

Make the Pumpkin Cream Cheese Frosting:
While the cupcakes are cooling, prepare the frosting. First, dry out the pumpkin puree by spreading it on a plate with paper towels to remove excess moisture until you have ¼ cup of dry pumpkin. In a large bowl, beat the softened butter with an electric mixer until pale and fluffy, about 5-10 minutes. Add the cold cream cheese and mix until fully combined. Then add the dried pumpkin and pumpkin pie spice and mix again. Gradually add the powdered sugar, one cup at a time, mixing until smooth and fluffy.

Assemble the Cupcakes:
Once the cupcakes are completely cool, transfer the frosting into a piping bag fitted with your preferred tip. Pipe a generous swirl of frosting onto each cupcake.

Serve:
Serve immediately, or store leftovers in an airtight container in the fridge for up to three days.