I bought a Betty Crocker cookie mix and used "I Can't Believe it's Not Butter". The cookies didn’t work at all! Was it because I used "I Can't Believe it's Not Butter"?
I love that the Betty Crocker web site says “Aside from taste and texture, butter and margarine are about the same when baking”. Well, if not for taste and texture, why eat a cookie?!
Of course, the site goes on to explain that margarine should be at least 80% vegetable oil. By law, margarine (and butter) must contain 80% fat. Spreads, like the one you used, can contain far less. Fat is an important part of what gives baked goods their structure, and a big part of what helps them brown. So when you use a spread with a lot less fat, it makes sense that the cookies won’t turn out right.
For more tips, see Top 10 Cookie Baking Secrets and Tips.