Preheat oven to 350 degrees. Line 2 12-cup muffin tins with parchment liners.
In a large bowl, beat the oil, brown sugar, and regular sugar for 2-3 minutes until combined. Whip in the eggs, one at a time, then the vanilla and yogurt. Add the flour, cocoa powder, dark cocoa powder, baking powder, baking soda, espresso (if using), and salt. With the mixer running, stream in the Guinness until the batter is just combined. Do not overmix.
Fill each cupcake tin 2/3 of the way with batter. Depending on what your heart says 2/3 of the way full is, you will get 22-24 cupcakes. Bake for 25-30 minutes until an inserted toothpick emerges with just a few moist crumbs. Cool 10 minutes in the pans, then transfer the cupcakes to wire racks. Cool completely.
Make the frosting. Whip the butter, Bailey's, and vanilla until combined, then add the powdered sugar. Crank the mixer to high, and beat until very light and fluffy, about 2-3 minutes. You may need an extra tablespoon of two of cream to achieve the desired texture. Scrape down the sides of the bowl with a spatula, as needed, to properly incorporate all the ingredients.
Pipe or frost each cupcake with the buttercream. Finish with a sprinkle of instant espresso, as desired.