As soon as you complete The Spire's Secret quest, you will likely be given this single-part quest. For me, completing the previous quest and attaining Level 42 happened at the same time, so it's possible that if you complete the previous quest, and aren't yet at Level 42, you may need to wait until you get to Level 42 before Magnus gives you this quest.
In addition, there was something odd about this quest. I don't know if it's a glitch or what. As you'll see below, one of the objectives of this quest is to complete work on an Ice Pond. However, the picture shown on my screen was one of an Icy Treats Shoppe, and the objective was already checked off for me. Given that I already had an Ice Treats Shoppe at that point, but did NOT have an Ice Pond, I can only conclude that the programmers goofed somewhere. Point being, you may get a pass on parts of this quest, but you'll likely want to have an Ice Pond anyway to complete it, because Ice Cream does require a LOT of ice!
Complete work on 1 Ice Pond to get extra ice
Make 3 Coconut Ice Cream
Make 3 Mystic Ice Cream