Add chocolates, mini marshmallows, and milk to a large, heavy saucepan.
Cook over low heat, stirring constantly, until melted and well-combined.
Remove from heat, and cool completely. This is key. If the chocolate mixture is still warm, it will melt the whipped cream.
In a large bowl, beat whipping cream, sugar, and vanilla together to soft peak stage.
Reserve 3/4 whipped cream to top mousse for serving.
Cover, and chill reserved whipped cream until ready to serve.
Fold 2 cups whipped cream into cooled chocolate mixture until well-combined.
Divide mousse among dishes, and chill for at least 30 minutes, or until ready to serve.
Serve topped with reserved whipped cream.
Refrigerate any leftovers.
Notes
Chocolates: This chocolate mousse recipe isn't overly sweet. If you prefer more of a dark chocolate flavor, use all bittersweet in place of the semisweet chocolate. If you'd like sweeter mousse, try using all semisweet chocolate or substituting part milk chocolate.
Milk: You can use any milk from skim to whole milk. Using whole milk will give you richer, creamier mousse.
Whipping cream: If you'd like more tips about making whipped cream, you can find more information here.
Sugar: If you like sweeter whipped cream, increase the sugar to 1/3 cup.