Line a 12 cupcake pan with standard size liners. Set aside.
In a small sauce pan melt the butter and add the 2 cups of oatmeal stout. Bring to simmer, add the cocoa powder, whisk until well incorporated and remove from heat. Set aside.
Heat oven to 375°F.
In the bowl of a stand up mixer beat the eggs with the sugar.
Add the salt, flour, baking soda, baking powder and vanilla and stir a few times to combine.
Begin to slowly add the stout butter mixture and continue to mix on low until all the ingredients are well combined. The batter will appear runny. Do not despair.
Pour the batter into the cupcake liners and bake for 16-18 minutes.
Meanwhile make the frosting.
Frosting: Whisk the Mascarpone and confectioners sugar and add the cocoa (if using) and vanilla. Blend well and set aside.
In a separate bowl, whip the heavy cream until it begins to stiffen slightly. Fold it into the Mascarpone mixture.
Let the baked cupcakes cool off before decorating with the frosting.