Combine about half of the chocolate, the brown sugar, cocoa powder, and salt in a medium sauce pan, mixing well. Stir in cream and corn syrup. Warm the mixture over moderate heat, stirring until the chocolate melts.

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, along with the butter and vanilla extract. 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.