In large saucepot, over medium heat, warm together milk, cream, and ½ cup of the sugar, until sugar dissolves. Do NOT allow mixture to boil.
Whisk in cocoa powder and chopped chocolate, whisking until mixture is free of lumps.
Meanwhile, in medium mixing bowl, whisk together egg yolks and remaining sugar, until thick and eggs pale in colour.
Ladle a small amount of the warm milk mixture to eggs to “temper.” Then slowly whisk eggs into saucepot of milk and cream. Add coffee and continue stirring over medium-low heat until temperature reaches 175-180 degrees F. Again, do NOT allow mixture to boil.
Remove from heat, add vanilla extract and pour custard through a sieve into clean bowl and set in fridge to cool completely.
Once cooled, pour mixture into ice cream machine and churn according to manufacturer’s directions.
Transfer to container with tight lid and freeze until ready to eat.
To serve, divide gelato between small serving bowls and top each with generous dollop of marshmallow fluff.