Combine first 4 ingredients in a small saucepan and bring to a simmer. Cook for 5 minutes. Turn off heat, cover and steep for 15 minutes. Strain solids and return the liquid to a clean saucepan. Whisk in cocoa powders and corn syrup until smooth. Simmer for 2 minutes. Turn off heat and stir in chopped chocolate until melted. Cool to room temperature then refrigerate.
Serve drizzled over ice cream.