01 - Preheat oven to 350°F (175°C). Line a 9×13-inch baking dish with parchment paper or lightly grease it.
02 - In a bowl, combine graham cracker crumbs and melted butter. Press the mixture firmly into the bottom of the prepared pan to form a crust.
03 - Evenly spread the chopped rhubarb over the crust.
04 - Drizzle the sweetened condensed milk over the rhubarb layer.
05 - Sprinkle white chocolate chips and walnuts (if using) evenly over the top.
06 - Finish with a layer of shredded coconut.
07 - Bake for 30–35 minutes, or until the edges are golden and the center is set.
08 - Cool completely in the pan before cutting into bars.