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