Place 1 1/2 cups of the chocolate chips into a glass bowl and melt by microwaving in 30-second intervals, stirring in between each interval.
Place 12 paper muffin cups in a muffin tin. When the chocolate is completely melted and smooth, use a small pastry brush to coat the bottom and sides of the paper muffin cups.
Refrigerate for about 10 minutes and then add a little bit of coating to any thin spots. Chill for at least 2-3 hours before removing the paper.
To make the mousse, combine the marshmallows and milk in a saucepan and stir over low heat until the marshmallows are melted.
Remove from heat and add 1 cup of semi-sweet chocolate chips. Stir until melted and cool completely.
Use an electric mixer to beat the heavy cream until stiff and then fold the whipped cream into the chocolate marshmallow mixture.
Scoop into the chocolate cups right before serving. Top with a berry if desired! Store any extra cups and mousse in the refrigerator until ready to serve.