Preheat oven to 325°F Prepare two 12-count cupcake pans by lining them with paper liners.
Melt chocolate in the top of a double boiler (with simmering water in the bottom part of the double boiler), stirring occasionally. Once it’s smooth remove the bowl from the heat, and let chocolate cool for 5 to 10 minutes.
In a medium sized bowl sift together flour and baking soda, set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream butter and sugars together until smooth. Add the sour cream then mix in the eggs one at a time, beating well after each addition. Add buttermilk, vanilla extract and melted chocolate and mix to thoroughly combine. Add the dry ingredients in thirds, mixing well after each addition.
Using a medium sized ice cream scoop fill prepared pans with one full scoop each. Bake 15 to 20 minutes or until a toothpick inserted into the center of a cupcake comes out clean. Remove pan from the oven and set on a rack to cool.
For the buttercream:.
In the clean bowl of a stand mixer fitted with a paddle attachment beat together butter, powdered sugar, shortening, milk and vanilla. Blend until smooth and creamy. Add food coloring as desired and mix until color is consistent throughout.