Preheat oven to 350^.
Combine graham cracker crumbs, butter and sugar in a large bowl; stir until well mixed. Press into a 9-inch glass plate to form a crust. Bake 10 minutes; transfer to a wire rack and cool completely.(you may use a store bought pie shell if desired.)
Place chips in a small microwave-safe bowl. Microwave at medium power for 30 second intervals until melted, stirring after each interval. Stir until smooth. Combine chips and sweetened condensed milk. mascarpone, lime juice and lime zest; whisk or beat with an electric mixer until smooth. Pour into cooled crust; smooth top. Chill at least 4 hours; or place in freezer for 15 minutes just before serving. Slice pie into 8 wedges. Serve with dollops or whipped topping. Garnish with toasted coconut and lime slices.