Preheat the oven to 350 degrees and line a cupcake pan with cupcake liners.

In a small bowl whisk together the flour, black cocoa powder, baking powder, baking soda, salt and espresso powder. Then set aside.

Cream the butter and sugar together in a large bowl with an electric mixer on high speed until fluffy, about 2-3 minutes.

Then add in egg, egg yolk and vanilla and mix on medium-high speed until pale in color and smooth, about 1 minute.

Next mix in the milk and sour cream on low speed until combined.

Add the dry ingredients to the wet ingredients and combine on low speed.

Divide the batter evenly among the 12 liners, each should be about three-quarters full. Bake the cupcakes for 17-19 minutes.

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

While the cupcakes are cooling make the frosting by adding the softened, unsalted butter and salt to a medium sized bowl. Mix the butter on high speed until it is pale in color and fluffy, about 5-10 minutes.

Then sift in the black cocoa powder. Mix until combined. Then sift in the powdered sugar a little at a time until it is all combined.

Add in the heavy cream and mix until the frosting is light and fluffy, about 1-2 minutes.

Once the cupcakes are cool, add the black cocoa buttercream to a large piping bag fitted with a decorative star tip. Pipe a generous amount of frosting onto the tops of each cupcake and add a few sprinkles if you wish.