This one threw us a curveball, though, as we found the graham cracker crust to be tricky. It needs fat to help bind the crumbs and to maintain a crisp texture when baked, filled, and chilled. Too little fat delivers a crumbly mess that grows soggy after sitting—as we discovered to our chagrin in our first few tests. Nor did canola oil, in place of butter, yield the crisp goodness we were looking for. The breakthrough discovery, oddly, was white chocolate. It melts to bind the crumbs in the oven, then hardens when cooled to form a perfectly crisp crust. Fat-free condensed milk and 2% Greek yogurt offer a light, tangy, creamy filling with lots of zesty, fresh-squeezed lime juice. Fat-free whipped topping adds a fluffy finish with a lot less fat, of course, than cream. This pie boasts 75% less fat than the original and makes this Key lime treat a dessert you can enjoy all year long.