Preheat the oven to 350°F and line a standard cupcake pan with 12 cupcake liners.

In a small bowl whisk together the flour, cocoa powder, baking powder, baking soda and salt. Set aside the dry ingredients.

In a large bowl, cream together the butter and granulated white sugar with an electric hand mixer on high speed until fluffy, about 2-3 minutes. (If using the bowl of a stand mixer, use the paddle attachment.)

Next add in the egg, egg yolk and vanilla and mix on medium-high speed for 1 to 2 minutes until pale and smooth.

Add in the sour cream and milk and mix on medium-low. (The mixture will look curdled at this point, but it will pull together once the dry ingredients are added.)

Add in the flour mixture and mix on low speed, just until the batter is combined and smooth, scraping the sides of the bowl as necessary.

Divide the chocolate cupcake batter among each liner. Fill each cup two-thirds to three-quarters full.

Bake for 17-20 minutes, or until a toothpick/cake tester comes out clean from the centers with just a few moist crumbs.

Let the cupcakes cool in the pan for 10 minutes, then transfer to a cooling rack to cool completely.

Add the peanut butter chips to a medium bowl.

Heat the heavy cream in a small saucepan, on the stove over medium heat just until steaming.

Pour the hot cream over the peanut butter chips. Stir to combine the melted peanut butter chips and cream together. Let the ganache cool to room temperature, stirring as it cools. (It will thicken once cooled, and as it is stirred.)

Once cooled, transfer the peanut butter ganache to a piping bag.

Add the butter a large bowl. Whip with an electric mixer on high speed for 5 to 10 minutes until the butter is pale, fluffy and has doubled in size.

Add in the cream cheese and mix on medium speed until combined.

Sift in the powdered sugar a little at a time, mixing on low speed until all is combined.

Add the peanut butter to the frosting and combine on low, then high speed until light and fluffy, 1-2 minutes. Transfer the frosting to a piping bag fitted with a decorative tip.

Once cooled, remove the center of each cupcake using a cupcake corer.

Fill the middle of each cupcake with peanut butter ganache. Then pipe the peanut butter frosting on top of the cupcakes.

Top each with mini peanut butter cups and serve!