Preheat your oven to 400°F (200°C). Rinse the chicken breasts under cold water and pat them dry with paper towels. Season both sides with salt and pepper.

In a medium saucepan, combine the Guinness, brown sugar, soy sauce, apple cider vinegar, Dijon mustard, garlic powder, and onion powder. Stir over medium heat until the sugar dissolves completely and the mixture is well combined.

Place the chicken breasts in a baking dish and pour half of the Guinness glaze over them, ensuring the chicken is evenly coated.

Place the baking dish in your preheated oven and bake for 20 minutes. After 20 minutes, take the chicken out and flip it over, brushing additional glaze on top.

Switch the oven to broil and cook for an additional 5-7 minutes, or until the chicken is cooked through and has a nice caramelized glaze. The internal temperature should reach 165°F (75°C).

Once cooked, let the chicken rest for a few minutes, then slice and garnish with freshly chopped parsley before serving.