FOR THE CAKE:
Preheat oven to 350 degrees.
Place paper cupcake filters in pan and set aside.
Bring the stout and butter to a simmer in a large heavy saucepan over medium heat. Add cocoa powder and whisk until mixture is smooth.
Cool slightly.
Whisk flour, sugar, baking soda and salt in a large bowl.
Using an electric mixer, beat eggs and sour cream on medium speed. Add stout mixture to egg mixture and beat just to combine.
Add flour mixture, slowly, and beat on slow speed briefly.
Using a spatula, fold batter until completely combined.
Using an ice cream scoop, fill cupcake liners 3/4 of the way full.
Bake cupcakes for 25-30 minutes (for large cupcakes) 20-25 minutes for regular sized, or until cake tester comes out clean.
Transfer cupcakes to cooling rack and let cool at least 20 minutes before frosting.
FOR THE FROSTING:
Bring cream to a simmer in heavy medium saucepan. Remove from heat. Add chopped chocolate and whisk until melted and smooth.
Transfer frosting to a bowl and cool completely, stirring every 5 minutes until thickened and a spreadable consistency.
Frost cupcakes and store in air tight container.