PREHEAT oven to 350° F. Combine graham cracker crumbs, butter and granulated sugar in a large bowl; stir until well mixed. Press into a 9-inch glass pie plate to form crust. Bake for 10 minutes; transfer to a wire rack and cool completely.**
PLACE morsels in a small microwave-safe bowl. Microwave at MEDIUM (50%) power for 30-second intervals until melted, stirring after each interval. Stir until smooth. Combine morsels with sweetened condensed milk, mascarpone, lime juice and lime zest; whisk or beat with an electric mixer until smooth. Pour into cooled crust; smooth top. Refrigerate at least 4 hours. (Optionally, place in freezer for 15 minutes just before serving). Slice pie into 8 wedges. Serve with dollops of whipped topping or whipped cream. Garnish with toasted coconut and lime slices, if desired.
*Fresh (not bottled) Persian lime juice may be substituted.
**1 prepared 9-inch (6 oz.) graham cracker crust may be substituted if desired.