Prepare (either use spray or butter and flour) (4) 8-inch round cake pans. Prepare cake mix, following directions on package. Pour the mixture evenly between the four pans. Bake according to the package directions. When cake is done, put layers separately on racks to cool.
Heat butter in a large skillet over medium heat. Add the chopped apples and brown sugar, cook until apples are tender, stirring frequently, approximately 10 minutes. In a small bowl, stir cornstarch and water until smooth; add this mixture to apples.
Bring to a boil; cook and stir until thickened, 1 to 2 minutes. Stir in walnuts and cinnamon. Remove from heat and cool completely.
To assemble cake, place one cake layer on a plate, spread with 1/3 of the apple mixture, top with the next cake layer, spread with 1/3 of the apple mixture, top with the third cake layer, and spread with the remaining apple mixture. Put 4th cake layer gently on top.
In a large bowl, combine all the frosting ingredients and beat until smooth. Frost the sides and top of the cake. Enjoy!
