1 tablespoon coffee liqueur, or strong brewed coffee
2 teaspoons pure vanilla extract
1 1/2 cups Marshmallow Fluff
Combine 2 1/2 cups milk, corn syrup and brown sugar in a large heavy saucepan. Heat over medium heat, stirring to dissolve the sugar, until steaming.
Meanwhile, whisk together egg yolks, cocoa, cornstarch and the remaining 1/2 cup cold milk in a medium bowl until smooth.
Gradually whisk the hot milk into the egg-yolk mixture; then return to the pan. Cook over medium heat, whisking constantly, until the mixture boils and thickens, about 3 minutes. (Because the custard is thickened with cornstarch, it will not curdle when it boils.)
Remove from the heat, add chocolate, and stir until melted. Stir in coffee liqueur or brewed coffee and vanilla.
Transfer the custard to a large clean bowl and place a piece of wax paper directly over the surface to prevent a skin from forming. Cool completely. Add Marshmallow Fluff and mix with a whisk until as smooth as possible. (The mixture will be a little lumpy; lumps will break down during stir-freezing.) Chill until cold.
Pour into the canister of an ice cream maker and freeze according to the manufacturer's instructions.