Wait for at least 15 minutes before sprinkling the Pop Rocks over the hardening candy. Test one corner by sprinkling some of the pop rocks over the bark. If it starts fizzling and popping wait another five minutes and test again. When you can sprinkle them on without them fizzling and popping, sprinkle all of it over the top. Let it firm completely before cutting or breaking into pieces. Store loosely wrapped at room temperature.