I've been struggling with another brand's edible markers for a few years, never able to get the thin line I wanted for fine details. Eureka! These Rainbow Dust markers are a game-changer for me.
US Rainbow Dust Double Sided Edible Pen Full Set
These have the smallest tip that I have been able to find on an edible pen. I love that it writes like a pen or pencil, creating perfect lines for fine details. You also have the thicker end to add to its versatility. I did have one pen arrived that had leaked (I've ordered two sets so far), and it was quickly replaced.