Roll them completely in cinnamon sugar, not sprinkle. The coat dries the surface into a shell, and the crackle is that shell splitting as the inside keeps rising.
Which means coverage is the whole technique. A patchy coat dries patchily and gives you a patchy shell, and cooks who sprinkled instead of rolling lost the edges and the centres together. The coat is also the line between this and a sugar cookie: cinnamon in the roll is the single clearest thing that makes it a snickerdoodle.
Read against 208 published snickerdoodles recipes and 1,672 reports from cooks who changed one thing.
Coverage first, then crystal size.
The coat has to cover the surface completely, because what it does is dry that surface evenly.
Cooks who sprinkled cinnamon sugar over the top instead of rolling the ball in it reported failures at both the edges and the centre, which is exactly what an uneven shell would produce. Get a bowl, get the ball fully coated, and accept that this takes an extra thirty seconds. It is the single step that decides whether the top cracks.
Big raw-cane crystals sit on the surface instead of drying it, and they fail the same way sprinkling does.
The coat works by pulling moisture out of the outer layer of dough, and it can only do that where it is actually in contact. Coarse crystals touch in fewer places and leave gaps between, so the shell forms unevenly. Save the sparkly sugar for something where it is doing a decorative job.
Canon's ratio: half a teaspoon of cinnamon per quarter-cup of sugar to start, and up to equal parts if you want it to dominate. A ninefold overdose is survivable, and you will taste nothing else.
Uncoated cookies from the same batch kept their chew entirely.
That is about as clean a control as this site gets: same dough, same oven, coat as the only difference, and the inside was unchanged. So if your snickerdoodles are wrong in the middle, the coat is not the culprit and rolling them more thoroughly will not fix it. Look at the cream of tartar, which is the ingredient that owns the crumb.
Four things at once, in the same moment.
Sugar pulls moisture, so the coat dries the outer millimetre of dough into a shell before the inside has finished expanding. The inside keeps rising, the shell cannot stretch with it, and it splits: that split is the crackle, and it is a structural event rather than a cosmetic one.
At the same time the surface sugar caramelises on its own terms, which needs no protein, while the dough underneath browns by the ordinary route that does. Drying, splitting and browning are one moment rather than three, which is why a cookie with a good crackle almost always has good colour too, and why chasing the colour with extra time costs you the cookie rather than improving the top.
So you know how far to trust the rest of this page.
The drying-shell-and-rupture chain comes from baking science and is well established. That coverage matters, and that coarse crystals fail, comes from cooks reporting their own results, and there are not many of them. They agree with the mechanism, which is the best kind of support short of a trial.
The same-batch uncoated comparison is the closest thing to a controlled test in this dish's file, and it is a single report. We lean on it because it is unusually clean, not because it is repeated.
Our recipes are resolved for how they handle the fat, the rest and the shaping, not for how thoroughly they coat. So this page is mechanism and cook reports rather than a comparison across the corpus.