Bake cake in 2 round layer pans, 8 or 9x1 1/2 inches.
Split to make 4 layers.
Reserve 1 tablespoon orange juice concentrate for glaze.
Fill layers with remaining concentrate, using 3 tablespoons between each layer.
Mix reserved 1 tablespoon concentrate, the butter and confectioners' sugar.
Spread over top of cake.
Chill thoroughly before serving.