The dense base layer is like a rich, fudgy brownie, so don't overcook it or the dessert bars will be dry. Refrigerating the mint bars allows the chocolaty top layer to set properly. You can make the dessert up to one day ahead. For a more grown-up taste, you can also use dark chocolate chips for some or all of the semisweet chocolate chips in the glaze.
View Recipe: Chocolate-Mint Bars