Look for shiny, heavy gauge cookie sheets with very low or no sides. Avoid dark cookie sheets. They may cause cookie bottoms to overbrown. Use jelly-roll pans (15x10x1-inch baking pans) only for bar cookies. Other types of cookies will not bake evenly in a pan with an edge. If you must use such a pan, turn it over and bake cookies on the bottom. Insulated cookie sheets tend to yield pale cookies with soft centers. You may have trouble using them for cookies containing a large amount of butter, such as shortbread, because the butter may melt and leak out before the dough is set. Do not bake cookies on insulated cookie sheets long enough to brown on the bottom because the rest of the cookies may become too dry.