Preheat oven to 350°F (175°C). Line an 8x8 inch pan with parchment paper, leaving an overhang on two sides.

In a bowl, mix graham cracker crumbs, melted butter, and sugar until it resembles wet sand.

Press the crumb mixture firmly and evenly into the bottom of the prepared pan. Bake for 10 minutes.

While crust bakes, make the filling. In a large bowl, whisk together the sweetened condensed milk, 1 cup unsweetened coconut, 1/2 cup sweetened coconut, lime juice, and lime zest until combined. The mixture will be very thick.

Pour the thick filling over the warm, pre-baked crust and spread into an even layer.

Sprinkle the remaining 1 cup of sweetened coconut evenly over the top.

Bake for 25-30 minutes, until the edges are bubbling and the top is golden brown.

Let cool completely in the pan on a wire rack (about 1 hour). Then, cover and chill in the refrigerator for at least 2-3 hours, or overnight, until firm.

Once chilled, use the parchment overhang to lift the bars out. Cut into 16 squares. Garnish with lime slices if desired.