Preheat oven to 350 degrees F. Line a muffin tin with paper cupcake liners and set aside.
Bring beer and butter to a boil in a small saucepan, over medium-high heat. Stir.
Remove from heat and add cocoa powder. Mix.
In a large bowl, whisk together flour, sugar, baking soda and salt.
Add butter and beer mixture into dry ingredients and mix.
Add egg and sour cream and mix.
Scoop cupcake batter into lined muffin tin, filling each section about 2/3 of the way.
Bake for 15 to 17 minutes. Allow to cool on a wire rack before decorating.
To make the buttercream:
Using a mortar and pestle (or a food processor), break down peanut into small bits.
In a large bowl, use an electric mixer to beat together butter, confectioners’ sugar, peanut butter and milk. Start on low speed and gradually increase to high.
Once the ingredients are mixed and smooth, scoop into a piping bag with Wilton tip #12. Frost each cupcake.
Sprinkle each frosted cupcake with peanut bits. Serve and enjoy!