Preheat oven to 350F and line a 12 cup muffin tin with muffin cups.
In the bowl of a stand mixer with the paddle attachment, beat the butter, sugar and brown sugar until light and fluffy. Add the egg and vanilla extract, and Greek yogurt and beat to combine.
With the mixer running on low, add the Guinness slowly. Once it is incorporated, add the baking soda and cocoa powder and incorporate well. Finally, add the flour and mix, scraping down the sides of the bowl to ensure everything is mixed well.
Spoon batter into the muffin liners, filling them about 3/4 full. Bake for approximately 20-22 minutes or until the toothpick inserted into the center of a cupcake comes out clean. Transfer to a cooling rack and let cool completely.
For the frosting:
In the bowl of a stand mixer with a paddle attachment, cream together the butter, cream cheese, and confectioner's sugar until light and fluffy. Slowly add as much milk as needed to give it a nice consistency. Transfer to a piping bag and pipe onto cupcakes.