If vanilla is the cookie's soul, then flour is its breath, or what causes the cookie to inflate like a lung once it hits the heat of the oven. What type and how much flour you mix into your batter will ultimately affect your cookies' structure and texture.
Biochemist and cookbook writer Shirley Corriher notes that it's the protein in the flour that grabs moisture and defines the dough's consistency. Higher-protein flour will absorb more moisture, and you'll wind up with drier dough and tougher cookies. Lower-protein flour will result in wet dough and flatter cookies [source: Corriher]. So, for example, Corriher recommends adding one-fourth of a cup more all-purpose flour to the Nestle Toll House chocolate chip cookie recipe than it calls for. This increases the protein content and prevents the dreaded cookie spread.
Flour is a delicate thing, and it needs to be moved and measured carefully. You should stir up the flour a bit before you dip it from your container to your dry measuring cup. Don't heap or pack the flour into the cup. Just spoon it. And when you've filled the measuring cup, scrape the top edge with a spatula or a knife to even it out. Resist the urge to tap the measuring cup on the counter to level out the flour [source: Better Homes and Gardens]. That's not necessary.