In a large mixing bowl, mix together graham cracker crumbs and melted butter. Press into greased or parchment-lined 18 x 13 sheet pan and bake for 10 minutes. Take out of oven and let cool.
Using a mixer beat sweetened condensed milk, sour cream, and lime juice just until smooth. Stir in lime zest.
Stir in chopped cashews and coconut.
Pour mixture onto baked crust. Bake for 8-10 minutes until filling is set.
Cool completely before cutting cut into squares. Chill until ready to serve. Top with a swirl of whipped cream and additional lime zest if desired.