Definition: resting dough in the fridge before baking to firm the fat, control spread and deepen flavour.
Chilling is the single best thing you can do for a bakery-style cookie. Cold butter takes longer to melt in the oven, so the cookie sets before it spreads — which is where that lovely height and chew come from. There’s a flavour bonus too: a longer rest lets the flour hydrate and the sugars mingle, so the baked cookie tastes rounder and more toffee-ish.
In practice: 30 minutes firms things up; overnight is even better for flavour. Scoopbefore chilling so you’re not wrestling with rock-hard dough.
Related:creaming,bicarbonate of soda.