Set aside a medium saucepan and a medium bowl that fits comfortably on top. Also set aside a baking sheet, four small bowls and four 5-inch balloons.
Fill the saucepan with 2 inches of hot water and place on low heat.
Chop the white chocolate into pieces. Add to the medium bowl and place on top of the saucepan. Let the chocolate melt until smooth, stirring occasionally. (If the water starts to boil at any point, turn the heat off.) Remove from heat and set aside.
Add the chopped semisweet chocolate to a second bowl and follow the same steps to melt until smooth.
Meanwhile, arrange the four small bowls on the baking sheet. Also inflate the balloons so they are approximately 6-inches in length.
Pour the melted semisweet into a small ziptop bag and seal. Then use scissors to clip 1/8-inch off corner off the bag. Pipe the semisweet chocolate in a zigzag pattern into the melted white chocolate.
Dip a balloon gently into chocolate, about 3 inches deep. Then rotate 90 degrees and lift back out (this creates a swirl pattern). Place upside down in the bowl on the baking sheet to cool. Repeat for remaining balloons.
Place baking sheet in the freezer for 60 minutes to let the chocolate harden. Then use a knife tip or toothpick to prick each balloon so it pops. Gently peel off the remnants of the balloon, placing the finished ball on the baking sheet. Freeze until ready to serve.
To serve, melt semisweet chocolate until piping hot. Meanwhile, arrange ice cream or another small dessert on serving plates. Place the frozen chocolate ball on top to cover.
Pour the melted chocolate onto each ball in a criss-cross pattern to reveal the dessert underneath. Enjoy immediately.