Grease 2 layer cake pans (each 9x1.5") line bottom with wax paper.
In a small sauce pan over low heat , heat milk for about 5 minutes ; remove from heat ; add coconut and let stand for about 15 minutes. Pour mixture in a strainer over a small bowl , drain well , pressing coconut against the side of strainer with a spoon ; reserving coconut milk.
On wax paper sift together , flour , cornstarch , baking powder and salt ; set aside.
In a large mixing bowl cream butter and sugar ; thoroughly beat in egg yolks , 1 at a time (put egg whites in a medium mixing bowl and set aside ).
Sift flour mixture into creamed mixture in 4 additions , alternately with the coconut milk , ending with the flour mixture and mixing until smooth after each addition.
With clean beaters whip egg whites and cream of tartar , until very stiff straight peaks form when beater is slowly withdrawn.
Gently fold egg whites into batter , blending well.
Turn batter into prepared pans. Bake in preheated 350° oven until cake springs back when lightly touched--30-35 minutes.
Place pans on wire rack to cool for 10 minutes ; with a small spatula loosen edges ; turn out on wire rack ; remove paper ; using another wire rack flip cake over for right side up. Cool Completely.
Cut each layer into 3rds. horizontally (stack layers ) cake will have 6 of them.
Spread 1/4 cup chilled Coconut Custard between each layer ; spread the remaining coconut custard over the sides and on top of the cake.
Sprinkle top with 1/3 cup of toasted coconut , reserved from Coconut Custard recipe ; Cover and store in the refrigerator.