Mascarpone-rich Italian cream cheese helps to make this a tender loaf, and bittersweet chocolate and crystallized lemon or orange peel combine to make it unusual. The lemon peel-chocolate combination will get your attention because it is different; the orange peel with chocolate is more of a classic pairing of flavors but is no less fine. Serve a slice of either version of this elegant loaf with a cup of rich coffee, and you are in for a treat. The crystallized peel is sold in bakery supply stores. Citron, which figures in many fruitcakes (to the dismay of some otherwise dedicated fruitcake eaters), is not a substitute.