Julienne's Graham Cracker Chewy Bars

Note: Adapted from "Celebrating With Julienne" by Susan Campoy. Campoy writes, "I adapted this recipe from one of my favorite books, 'Nantucket Open-House Cookbook,' by Sarah Leah Chase. I love watching people's expressions when they bite into these bars for the first time. The textures and flavors -- rich toffee, a chewy center and a crunchy bottom -- make everyone smile. The secret is not to over-bake the crust. Serve them at room temperature, so the flavors meld together."