Preheat the oven to 350 degrees Fahrenheit. Spray a 9 x 9 x 2-inch metal baking pan with nonstick cooking spray and line it with parchment paper, leaving a 1-inch overhang on either side. Set aside.
In a large bowl, whisk together 2 tablespoons vegetable oil, 6 tablespoons melted unsalted butter, ⅔ cup Guinness beer, and 1 ½ cups granulated sugar until well combined. Add in 2 large eggs and 1 teaspoon vanilla extract and whisk until combined.
In a separate bowl, whisk together 1 ¼ cup all-purpose flour, ½ teaspoon salt, ½ teaspoon baking powder, ½ cup cocoa powder, and ¼ cup dark cocoa powder.
Mix the flour mixture into the Guinness mixture until just combined. Gently stir in 4 ounces semi-sweet chocolate chunks.
Pour the batter into the prepared pan and bake for about 35-40 minutes until a toothpick comes out clean from the center. If you like your brownies more on the gooey side, bake for 30-35 minutes.
Meanwhile, prepare the Guinness ganache by adding 8 ounces semi-sweet chocolate chips to a large bowl.
Heat ¼ cup heavy whipping cream and ¼ cup Guinness beer in the microwave for 1 minute and 30 seconds, then pour it over the chocolate chips. Let it sit for 2-3 minutes before whisking until combined and glossy. Cover the bowl and chill for 15-30 minutes.
Use a hand mixer to beat the ganache until light and fluffy. Frost over the top of the completely cooled brownies. Slice into bars and serve.