Crust: Beat butter and sugar in a bowl until smooth;beat in egg and vanilla. Sift flour, baking powder and salt and mix into butter/sugar mixture until a stiff dough forms; press into bottom and 1/2-inch up sides of prepared pan. Bake until firm, about 15 minutes.
While crust is baking, in a saucepan over medium heat, mix rhubarb, sugar cinnamon and 2 tbsp water. Bring to boil, then reduce heat and cook until rhubarb is soft, about 10 minutes. In a cup mix 2 tbsp of water with cornstarch, add to saucepan, stirring constantly, until sauce thickens. Spread over baked crust.
Topping:In a medium bowl, beat egg whites to soft peaks. Gradually beat in sugar until stiff peaks form. Spread meringue over rhubarb layer. Sprinkle with coconut. Bake 10 minutes or until meringue is golden brown. Cool completely. Cut into squares.