Preheat oven to 350°F & line 12 muffin slots with silicone muffin liners.
Cream together brown sugar, cane sugar and butter. Mix in egg, greek yogurt and vanilla extract until smooth. Finally, mix in Guinness.
In a separate bowl, combine all purpose flour, cacao powder, baking soda, and salt.
Slowly add the dry ingredients to wet and mix until well combined.
Fill each muffin cup with batter
Bake for 24-28 minutes or until a toothpick comes out clean.
Let cool completely. To make frosting, use a hand mixer to cream the butter. Add in powdered sugar 1/2 cup at a time., then mix in Baileys and vanilla extract.
Frost the cupcakes, top with sprinkles (optional) and enjoy!