In a large pot over medium heat, cook the Guinness for 5-10 minutes until reduced by about 50%, concentrating the flavor to 3oz.

Preheat the oven to 350°F (180°C) and line a 9-inch square pan with foil. Heavily grease the foil with non-stick baking spray. Add butter and sugar to the reduced Guinness in the pot and cook on low heat until butter melts and sugar fully incorporates. This step helps create the chewy texture with a crinkly top. Let cool for 5 minutes, then transfer to a mixing bowl.

Stir in the cocoa powder and vegetable oil into the cooled Guinness mixture, mixing until completely combined.

Whisk in the vanilla extract, egg, and egg yolk until smooth.

Mix in baking soda, salt, and cornstarch until combined. Gently fold in the flour until just blended, being careful not to overmix. Fold in the chocolate chunks just before all flour streaks disappear.

Pour batter into the prepared pan and bake for 25-27 minutes, or until a toothpick inserted comes out with moist crumbs but no wet batter. Avoid overbaking to preserve fudginess.

Allow the brownies to cool completely in the pan. Spread the Baileys Swiss Meringue Buttercream frosting with an offset spatula, creating swirls.

Cut into 9 to 16 pieces depending on sweetness preference. Store at room temperature or refrigerate in warm environments.