Warm 1 cup of the cream with the cocoa powder in a medium saucepan, and blend throughly using a whisk. Bring to boil, then reduce heat and simmer for 30 seconds at a very low boil, whisking constantly. Remove from heat and add the chopped chocolate. Stir until smooth, then stir in the remaining 1 cup cream. Pour the mixture into a large bowl and set a mesh strainer on top of the bowl.
In a saucepan, warm the milk, sugar, and salt. In a separate medium bowl, whisk together the egg yolks. Slowly pour the warm milk into the egg yolks, whisking constantly, then pour the warmed egg yolks back into the saucepan. Stir the mixture constantly over medium heat with a heatproof spatula. Scrape the bottom while stirring until the mixture thickens and coats the spatula. Pour the custard mixture through the strainer and stir it into the chocolate mixture until smooth. Stir in the vanilla and let cool.
Chill the ice cream mixture in the refrigerator, then process it in an ice cream maker according to the manufacturer’s directions.
To Make the Marshmallow Sauce, in a small saucepan, heat 1 1/2 cups Marshmallow Fluff and 3 tablespoons water over moderately low heat. Stir constantly until blended. Cool sauce. You can make sauce 4 hours in advance and store at room temperature, covered.