Place tempered dark chocolate in a cornet. The next step will be easier if you have an assistant. Have the assistant hold the balloon horizontally in front of you while you pipe chocolate lines from top to bottom all the way around the surface. When complete, drizzle more lines diagonally across the existing lines. Apply a bit more chocolate near the top opening and near the bottom to provide extra support. This will form a bird cage effect. Let the chocolate set. When the chocolate begins to harden, release a little bit of the air from the balloon, just enough so the balloon retracts slightly (about 1/8-inch from the outside). Carefully set the chocolate covered balloon aside. Depending on your room temperature, it will take a good 10 minutes to set.