Preheat your oven to 350 degrees. Line two muffin tins with paper liners and set aside.
Heat together the guinness and butter in a small sauce pan until the butter is melted. Whisk in the cocoa powder and stir until smooth. Remove from the heat.
Whisk together the flour, sugar, baking soda and salt in a small bowl. Set aside.
Combine the eggs, vanilla, and yogurt in the bowl of your electric mixer and stir until fully incorporated. Add the chocolate beer mixture and continue to stir until combined. Slowly fold in the dry ingredients. Fill your cupcake liners 3/4 full and bake 15 minutes or until a toothpick comes out clean. Cool completely.