Why Are My Cookies Flat? There’s no single culprit for flat cookies. But if your cookies are spreading in the oven, you could be making one of these common baking mistakes. (Here are some other common baking problems for cookies, bars and brownies.) Mistake 1: The butter is too soft Kitchens tend to heat up during any baking extravaganza, which means the butter you leave on the counter to soften might just get too soft. If this happens, the butter will melt faster in the oven and your cookies will flatten before they’ve been able to set. Thirty minutes is usually enough counter time to so...