Hi Amy! There are a number of reasons as to why cookies will spread like that. A few of the most common reasons are because the cookie dough is too warm before baking (the fat will melt faster, causing the dough to spread), the dough was not mixed thoroughly (leaving pockets of fat and sugar) or the butter and sugar were over creamed. It could also be caused by out-dated leavening agents or not enough flour in a recipe. We would recommend chilling the dough before baking (as outlined in this blog) and making sure to avoid over creaming your ingredients (this blog can help explain why).

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