Measure 1 tablespoon of the Guinness into a small bowl or cup. Set aside. Pour the rest of the bottle into a medium sauce pan and bring to a boil over moderate heat. Reduce heat to low and cook until reduced to about 2/3 of a cup, about 20 minutes.

In the meantime, combine half of the chocolate, the brown sugar, cocoa powder, and salt in bowl and mix well. Set aside.

Once the Guinness has reduced, combine chocolate mixture and corn syrup with the Guinness reduction in the saucepan. Warm the mixture over moderate heat, stirring until the chocolate melts completely.

When the mixture begins to steam, reduce heat to medium-low and continue cooking for about 5 minutes, stirring occasionally. Remove from heat and add the remaining chocolate, the butter, and reserved Guinness. Stir vigorously until smooth and glossy. Serve warm.

Store in a sealed container in the refrigerator for up to a week. To reheat, warm on the stove over moderate heat or in short bursts in the microwave. Stir frequently to avoid scorching.