For those who bake cookies on the regular, it's not uncommon to encounter the occasional oddball—one or more cookies from the same batch of dough, scooped out with all the rest, yet inexplicably larger, thinner, and browner than the rest. Sometimes these differences characterize the entire cookie; other times they affect one section alone.
There might only be a few such cookies in an entire batch, so it's easy to dismiss them as a fluke or some hot spot in the oven. But this sort of irregularity doesn't come from uneven heat, it comes from uneven textures within the dough—streaks of butter and sugar left from inadequate scraping of the bowl and beater during the creaming process.
As a quick refresher, the creaming method isn't about mixing ingredients together, it's about aerating them. Beating the butter and sugar, whether with a spatula or the paddle attachment of stand mixer, folds them together over and over, trapping little pockets of air with every turn.
This transforms the butter and sugar from a dense, dark, and gritty mass into something light, pale, smooth, and voluminous—the ubiquitous "light and fluffy" benchmark mentioned so often.
But if the bowl and beater aren't thoroughly scraped with a flexible spatula along the way, a dense film of butter and sugar may build up around the bowl, or clump in knots at the heart of a paddle attachment.
When the bowl and paddle are scraped along the way, these dense areas are less likely to form in the first place, and are soon homogenized into the batter when they do.
But without proper scraping to ensure these dense areas will have a chance to lighten, they'll create rogue pockets of unhomogenized dough that will spread, brown, and rise at a different rate than the rest—leading to those mysteriously malformed cookies.
As the creaming process itself represents a wide spectrum of potential textures, the exact degree of strange behavior can vary from a few cookies that are only a little thinner, denser, and browner than the rest, to those that spread and pool in an almost shocking way.
Along with scraping the bowl and beater as needed, thoroughly scraping the bowl and folding the finished cookie dough a few times with a flexible spatula can go a long way in homogenizing its texture.
Aside from sloppy technique, frequently experiencing dense streaks of butter and sugar shot through a dough or batter may be a sign the ingredients are too cold (as hard butter lacks the plasticity needed to aerate), or that the bowl to beater clearance of a stand mixer needs adjusting (as the paddle won't reach far enough into the bottom of the bowl).
The former can be avoided by paying close attention to ingredient temperature, and softening the butter to whatever stage or temperature is listed within the recipe; for recipes that don't include any information more specific than "room temperature" butter, it's generally safe to aim for something between 65°F/18°C and 70°F/21°C.
The latter can be addressed with a manual adjustment. For those using our recommended bowl KitchenAid bowl lift–style stand mixer, we have a guide to adjusting the bowl to beater clearance by hand. For those with tilt-head mixers, information on making this adjustment can be found in the instruction manual for the mixer, or on the manufacturer's website.
It's normal to wind up with a weird cookie from time to time, nobody's perfect and often that weirdo is someone's favorite crispy boi, but good technique and attention to detail can make these occasions rare indeed.