Place the Guinness in a small saucepan set over medium-low heat.
Bring the mixture to a simmer and cook it until it has reduced to about ½ cup. This should take 15-20 minutes. Make sure to stir occasionally so it doesn't bubble over. Once the Guinness has reduced, set it aside to cool completely before continuing.
Preheat the oven to 350 degrees F and line 12 muffin cups with paper liners.
In a large bowl, whisk together the vegetable oil, milk, eggs, and vanilla.
Then, whisk in ¼ cup of the cooled reduced Guinness.
Next, add the sugar and whisk to combine.
Add the flour, cocoa, and baking powder. Whisk just until no large lumps remain. You don’t want to overmix!
Divide the batter between the cupcake tins, filling each one ¾ of the way full.
Bake for 16-18 minutes or until a toothpick inserted into the center comes out clean.
Remove the cupcakes to a wire rack and cool completely before icing.