This is how I frost and decorate a double layer carrot cake. Just like you see in the bakery. I used cream cheese frosting with a few food colors and chopped walnuts to make this.
The recipe for the frosting I used:
8 oz cream cheese, softened
1/2 cup butter or margarine, softened
2 tsp vanilla extract
4 cups powdered sugar
Combine the cream cheese, butter and vanilla in a large bowl and mix until well blended. Gradually add in powdered sugar until fully incorporated.
I used LorAnn gel food coloring to color the frosting for the carrots.