Preheat oven to 375.
Line a 12 count cupcake pan with liners, set aside.
In sauce pan combine your stout and butter and heat on medium until melted, remove from heat and stir in your cocoa powder, let cool to room temp.
In bowl whisk together your flour, sugars, baking soda and salt.
In another bowl, with hand mixer beat together your milk and egg. Add in your cooled stout mixture until well blended.
Pour into the flour mixture and mix until just combined.
Pour into cupcake liners and bake for about 20 minutes or until when toothpick is inserted it comes out clean.
Remove from oven and let cool on wire rack.
Meanwhile to make your buttercream, beat together your butter and shortening until light and fluffy.
Mix in your salt and caramel until blended.
Once cupcakes are cool, add your frosting to piping bag fitted with an open start tip (or your favorite tip) and pipe onto cupcakes.
If desired drizzle with some extra caramel sauce, sprinkle with sea salt and top with pretzel.
One cup at a time add in your powdered sugar until desired consistency is reached, set aside.