Heat oven to 350°F.
Melt butter and chocolate in a microwave over medium heat; stir until smooth, then cool for 10 minutes.
In a bowl of an electric mixer, beat the eggs, sugar, water, sour cream and vanilla.
In a medium bowl, combine flour and baking soda; add to the egg mixture, mixing to combine.
Add the chocolate mixture and beat on high speed for 3 minutes.
Fill paper lined muffin cups two-thirds full. Bake for 15 to 20 minutes or until a toothpick inserted in the center comes out clean.
Cool for 10 minutes, then carefully remove from pan and allow to cool completely on a wire rack.
For the Frosting: melt the butter and chocolate in a microwave over medium heat; stir until smooth, then cool for 10 minutes.
With a hand held mixer or whisk, beat in the confectioners’ sugar, sour cream and vanilla on low until smooth.
Fill pastry bag with frosting and pipe decoratively on cupcakes.
Enjoy! [Leftovers? Store covered in the refrigerator.]