Make the coffee ice: Stir the sugar into the coffee until dissolved, then let cool. Pour into a 16-cube ice-cube tray and freeze until solid, about 3 hours.
Make the chocolate syrup: Combine the sugar and cocoa powder in a saucepan and slowly whisk in 2/3 cup hot water. Bring to a simmer over medium heat, whisking, until the sugar dissolves, 3 to 5 minutes; do not boil. Remove from the heat and stir in the chocolate, honey and vanilla until smooth. Transfer to a bowl and refrigerate until cool.
Make the frappes: Puree 8 coffee ice cubes, 1/2 cup milk, 1/3 to 1/2 cup chocolate syrup and 1 cup plain ice in a blender until smooth. Pour into 2 glasses, then repeat to make 2 more drinks. Top with whipped cream and more chocolate syrup.