In the top half of a double boiler, combine the 2 chocolates over simmering water.
Meanwhile in another small saucepan, heat the water to boiling and pour it over the fresh mint leaves in another bowl. Let steep while the chocolate is melting.
Using a sieve over a bowl, strain the mint water, reserving the water. Keep the water hot.
Stir the chocolate occasionally until melted; then whisk in the corn syrup and mint infused hot water (don't remove the double boiler from the heat). Whisk until smooth and shiny. (The sauce can be made up to 24 hours in advance and refrigerated. To rewarm, stir over low heat or heat in a microwave.)