For thick chicken breasts or if your chicken breasts are different in thickness, they need to be thinned to 3/4 inch thick. Without flattening, the surface of the breast will be overcooked and dry before the center reaches a safe temperature. To flatten, protect yourself and the surrounding area from splatter by placing the breast in plastic wrap or one-gallon freezer bags. Then use a meat mallet, rolling pin, or the bottom of a heavy pan on a large cutting board.