Prepare the carrot cake batter in a large bowl, then bake according to the package directions for a 9×13" cake. Allow the cake to cool, then cut it into cubes.

In a separate bowl and using an electric mixer, mix the butter and softened cream cheese, scraping down the sides as needed, until the mixture is light and fluffy. This will take about 2-3 minutes.

With the mixer on low speed, add the powdered sugar, ½ a cup at a time, until it is thoroughly mixed. Add the vanilla and beat the frosting on medium speed for about 2 minutes. Gently fold in the Cool Whip until it is completely mixed in.

In a trifle dish or glass dish, place chunks of carrot cake in the bottom of the bowl, then a layer of the cream cheese mixture. Sprinkle walnuts on top of the cream cheese. Repeat the layers until your trifle dish is full, ending with the walnuts on top.

Refrigerate until serving.