Preheat oven to 350 degrees F. Line or grease a cupcake tin and set aside.
In a medium bowl whisk together the flour, cocoa, baking soda, baking powder, and salt, combine and set aside.
In the bowl of a stand or hand mixer fitted with a paddle attachment combine the butter and sugar. Cream together for 3-4 minutes until light and fluffy. Add in the egg white and mix in on low for another 1-2 minutes. Add in the sour cream and vanilla and mix until combined. Slowly add in the milk.
Add in the dry ingredients and mix gently until incorporated. Using a scoop, fill each cupcake tin 1/2 to 2/3 of the way full. Bake in preheated oven for 13-15 minutes. The cupcakes should be slightly firm and a toothpick inserted should come out clean. Transfer to a cooling rack and let cool completely before frosting.
For the chocolate ganache:
In a small sauce pan, heat the heavy cream over medium until warm, 3-4 minutes. Place the chocolate in a mixing bowl and pour the hot cream over the top. Stir the mixture until completely melted. Transfer to the fridge to cool completely.
Using a small pairing knife core the middle of each cupcake, careful not to cut all the way to the bottom. Fill each cupcake middle with the cooled ganache.
For the chocolate buttercream:
In the bowl of a stand or hand mixer fitted with the paddle attachment cream the butter until light and fluffy, 4-5 minutes. Add in half of the powdered sugar and beat on low to incorporate. Add in the remaining powdered sugar, repeat. Scrape down the sides of the bowl, add in the cocoa powder and salt. Mix until smooth.
With the mixer on low, slowly add in the heavy cream, once combined add in the vanilla. If desired transfer to a piping bag fitted with a desired tip. Spread or pipe the frosting on each filled cupcake. Garnish with chopped or shaved chocolate.
Enjoy immediately or store frosted cupcakes in an airtight container in the fridge until ready to serve. Leftover ganache will keep in an airtight container for up to 1 week. Leftover buttercream with keep in an airtight container in the fridge for up to 2 weeks.
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