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 350F 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. 

Place the reduced Guinness and granulated sugar in a small saucepan set over medium heat. Cook until the sugar has dissolved (about 5 minutes) and then set the syrup aside to cool. 

Add the softened butter and melted chocolate to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until well combined– about 1 minute. 

With the mixer on low speed, slowly add the powdered sugar. Once all of the powdered sugar has been incorporated, add the reduced Guinness and vanilla extract. 

Scrape down the sides of the bowl and then turn the mixer to medium-high speed and beat until the buttercream becomes light and fluffy– about 1 minute. 

Transfer to a piping bag fitted with your choice of tip. 

To assemble, poke the top of each cupcake 5 times with a fork and pour over 1 teaspoon of the Guinness syrup. 

Pipe the frosting on top of each cupcake and garnish with chocolate shavings or sprinkles. 

Enjoy immediately and store any leftover cupcakes in the fridge for up to three days.