Preheat oven to 375 degrees.

Place parchment paper on 9x12 baking pan and butter the paper.

Set heatproof bowl on top of simmering water in saucepan. Melt dark chocolate, milk chocolate, white chocolate and butter in bowl. Once melted set aside to cool.

In mixing bowl, combine flour, cocoa powder, and salt.

Place eggs and sugar in stand up mixer and beat on high until fluffy.

Add in vanilla and cooled chocolate mixture. Beat on medium until fully incorporated.

Slowly add in flour mixture.

Slowly add the Guinness on low speed. I repeat slowly, you don't want the foam to overtake the batter.

Pour batter into pan. Use an icing spatula to even out the top.

Sprinkle a combination of the chocolate chips on top.

Bake for 25 to 30 min or until a toothpick comes out clean.