First, prepare the white chocolate shells. (We used silicone mini-muffin molds.) If you're using white chocolate bars, chop into 1/4-inch pieces and place in microwave-proof bowl. Place white chocolate pieces or chips in microwave and cook in 30-second intervals until chocolate is mostly melted. Remove and stir until completely melted. Place one heaping tablespoon of melted chocolate in a small plastic bag or a parchment cone and set aside. Spoon a heaping teaspoon of remaining melted chocolate in each mold, and use the back of a small spoon to coat entire opening up to the top. Place in freezer to set.
Meanwhile, make filling. Place a mixing bowl and beaters in freezer to chill. In a large, microwave-proof bowl, combine marshmallows or marshmallow cream and milk. Microwave for 1-2 minutes in 30-second intervals only until marshmallows are melted. (Mixture will puff up and fill bowl). Remove and stir until smooth. Stir in creme de cacao or other liqueur and mix thoroughly. Set aside to cool to room temperature.
Using chilled bowl and beaters, whip cream until nearly stiff. Fold in cream mixture. Spoon into chocolate molds all the way to the top, level and cover with plastic wrap. Freeze overnight.
For fruit sauce: If using frozen berries, place in strainer and set over large microwave-proof measuring cup to thaw. Some juices will collect in bowl. When thawed, press out juices with the back of a spoon. Discard fruit solids and seeds. Microwave juice until concentrated by half. Watch carefully – it will bubble up – and it may take 2-3 minutes or longer to reduce to a thick syrup. Add sugar to taste and stir until dissolved. Set aside to cool. When cool, place sauces in a small plastic bag or parchment cone.
To assemble: Chill serving plate. Pop out mousses from molds. Place on cold serving plate. To make flag, pipe red stripes and blue square. Use reserved white chocolate (melt in microwave but let it cool to room temperature to pipe) to make dots for stars. Or top mousse with any fresh fruit puree or piped dark chocolate.