Pour the Guinness beer in a small saucepan and simmer over medium-low heat, stirring often, until it has reduced down to approximately ½ cup. (approx. 15 minutes) Watch it carefully so it's doesn't bowl over.
Set the syrup aside to cool completely before continuing.
Preheat the oven to 350° F and line 12 cup muffin tin with paper liners.
In a large mixing bowl, add the oil, milk, eggs, and vanilla and whisk together well.
Then add ¼ cup of the cooled Guinness reduction from step 1.
Then add in the sugar and stir to throughly combine.
Next, add in the flour, cocoa, and baking powder and whisk just until there are no clumps (careful not to over-mix)
Pour the batter equally between the cupcake tins, filling each about 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.
This would be a good time to melt the chocolate chips for the frosting so it has time to cool.