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mix
medium-high, butter at 65°F
butter + 0.67 cup sugar, beaten until visibly paler and increased in volume — the sugar crystals are cutting air pockets into the fat, and that mechanical aeration is the only leavening that matters here. Butter warmer than about 70°F is too slack to hold the pockets and the cookies spread into wafers; butter straight from the fridge won't take them at all.