Preheat the oven to 375 degrees F. Line a 9 x 13-inch baking pan with foil and lightly grease.

Pour 2 cups of Guinness into a medium saucepan, and simmer on medium heat until it's reduced
down to 1 1/4 cups. Be sure to use a larger saucepan because the beer will foam. This should take 10-15 minutes. Keep a measuring cup handy to measure. Set beer aside to cool.

In a medium bowl,whisk together flour, cocoa, and salt.

Melt butter, bittersweet chocolate and white chips in the microwave, stirring every 30 seconds. Don't over-melt or the chocolate will burn. This should take about 90 seconds.

In a large mixing bowl or stand mixer beat eggs and sugar on high speed until light and fluffy, about 3
minutes. Add melted chocolates, beating until combined.

Add flour mixture in 2 portions, mixing between additions.

Whisk in the reduced beer mixture and the vanilla. Don't worry if the batter seems runny- this is what makes it so perfectly fudgey!

Pour into the prepared baking pan and scatter semisweet chocolate chips evenly over the top.

Bake 25-35 minutes minutes on center rack in the oven, until a toothpick inserted in the center comes out almost clean.

Let brownies cool in the pan about 10 minutes, then carefully lift edges of foil to remove from the baking pan and place directly on cooling rack.

Cool, uncovered, to room temperature.

Brownies can (presumably) last 3-5 days in refrigerated, airtight container. I've yet to witness that as my family devours them within 2 days.