Preheat oven to 350°F. Line 9x13-inch baking pan* with parchment paper.Grease and set aside.
Pour Guinness into a heavy bottom sauce pot. Bring a boil. Continue to boil over high heat for about 5 minutes until liquid has reduced to 6 oz (¾ cup).
In a large saucepan, melt butter and chocolate over medium-low heat. Once melted, add sugar and brown sugar. Stir to combine. Transfer to bowl of stand mixer fitted with paddle attachment. Let sit for 5 minutes to cool down.
With the mixer running on medium speed, add eggs one at a time, making sure each egg is fully incorporated before adding the next.
Whisk together flour, cocoa powder, and salt. Alternate adding dry mixture and reduced Guinness to the batter. Starting and ending with the dry mixture. Fold in chocolate chips.
Pour into prepared pan. Bake for 25-28 minutes until toothpick inserted in the center of the brownie comes out clean. Let cool on rack.