Preheat oven to 350 degrees and line a standard cupcake pan with 12 cupcake liners.

Add the granulated sugar and softened butter to a large bowl. Cream together with an electric mixer on high speed for 1-2 minutes until fluffy.(You can also use a stand mixer fit with a paddle attachment.)

Then add in the egg, egg yolk and vanilla and mix on medium speed until light and fluffy, about 1-2 minutes.

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

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

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

Fill each cupcake liner ¾ of the way full with batter. Bake the cupcakes for 17-20 minutes, or until a toothpick/cake tester comes out clean from the centers.

Let the cupcakes cool in the pan for 10 minutes then transfer to a cooling 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 with an electric mixer until the butter is pale in color and fluffy, about 5-10 minutes.

Add in the melted and slightly cooled dark chocolate and combine on medium speed.

Next sift in the Dutch process cocoa powder and powdered sugar, a little at a time. Mix on low speed until combined, then mix on high until fluffy.

Add in the heavy cream, a tablespoon at a time. Mix on high speed until the frosting is light and fluffy, about 1-2 minutes. (Add the heavy cream as necessary. If the frosting is light and fluffy after only two tablespoons, don't add the full amount. If it's still too dry after 3 tablespoons, add more.)

Once the cupcakes are cool, add the dark chocolate buttercream to a large piping bag fitted with a decorative tip.

Pipe a generous amount of frosting onto the tops of each cupcake. Serve right away.