Preheat the oven to 350 degrees F. Grease a 8- x 8-inch square baking pan.
Prepare Butter Crust. Bake crust for approximately 15 minutes or until firm. Remove from oven and set aside.
Prepare Rhubarb Filling. Spread prepared filling over the baked crust; set aside.
Prepare Meringue Topping. Spread prepared meringue over the layer of rhubarb sauce and sprinkle the grated coconut over the top.
Bake approximately 10 to 15 minutes or until the meringue is golden brown. Remove from oven and let cool completely before cutting into squares or slices.
In a medium bowl, cream together the butter and 1/4 cup sugar until fluffy. Beat in the egg and vanilla extract.
In another bowl, combine the flour, baking powder and salt. Stir into the butter mixture until a stiff dough forms. Press the dough into the bottom and 1/2 inch up the sides of the prepared baking pan.
In a large saucepan, combine the rhubarb, 1/2 cup sugar, cinnamon, and 3 tablespoons water. Bring to a boil and cook until the rhubarb is tender, approximately 10 minutes.
In a small bowl, mix the remaining 2 tablespoons water with the cornstarch; stir into the rhubarb mixture in the pan. Cook, stirring constantly, until the mixture has thickened. Remove from heat and spread over the baked crust.
In a large bowl, beat the egg whites together with the salt until peaks form. Gradually add the 1/2 cup sugar, continuing to beat the whites until stiff peaks form.
Rhubarb Meringue Cake Recipe: https://whatscookingamerica.net/cake/rhubarbmeringue.htm