I've been baking all my life, and there is one thing I've never been able to get right. It's bar cookies. I don't mean brownies or others like that. I do just fine with those. The problem is with ones like lemon bars, where you make a crust, bake it for a while, then put another layer on, and bake again. Somehow, the top and bottom never stay together when they're done. Any suggestions?