Add the softened butter and granulated sugar to a mixing bowl. Mix until light and creamy.
Add the egg, egg yolk, and vanilla, then mix until well combined.
Add the sour cream and mix until smooth.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture.
Add the milk and mix until a smooth chocolate batter forms.
Divide the batter evenly among the lined cupcake cups.
Bake at 180°C (350°F) for 18–22 minutes, or until a toothpick inserted into the center comes out clean.
Let the cupcakes cool completely, then make a small hole in the center of each cupcake.
Prepare the peanut butter ganache and spoon a small amount into the center of each cupcake.
Prepare the peanut butter cream cheese frosting and pipe it over the cupcakes.
Add a little peanut butter ganache to the center of the frosting if desired. Serve and enjoy.
