In a medium bowl, combine graham cracker crumbs, shredded coconut, brown sugar, and melted unsalted butter. Mix until the crumbs resemble wet sand. Press into the bottom of an 8x8-inch baking pan.

In a saucepan, whisk together sweetened condensed milk, coconut milk, whole milk, and cornstarch over medium heat. Cook, stirring constantly, until thickened, about 5-6 minutes.

Remove from heat and stir in vanilla extract, white rum, and additional shredded coconut until smooth. Allow to cool slightly before pouring over the crust.

Pour the filling onto the crust, spreading it evenly. Let cool at room temperature for about 20 minutes, then refrigerate for at least 4 hours until set.

Once set, cut into squares and sprinkle with toasted coconut if desired. Serve chilled.