In a small pot off the heat, whisk together the cocoa powder and sugar. Add the water and corn syrup. Bring to a boil while stirring. Simmer for 10 seconds.
Remove from the heat and add the chocolate. Mix until the sauce is creamy and smooth. Transfer to an airtight container and let cool to room temperature. The chocolate sauce will keep for 1 week in an airtight container at room temperature. Delicious with our potato doughnuts.