Add the flour, baking soda and powder, salt, and cinnamon. Mix well.
Fold in the carrots and nuts.
Spread the batter into the prepared baking pan.
Bake at 350 degrees Fahrenheit for about 55 minutes, or until the center of the cake springs back when you touch it.
Allow the cake to cool completely.
Make the frosting. Using an electric mixer, mix the butter and cream cheese, scraping down the sides as needed, until the mixture is light and fluffy. This will take about 3 minutes.
With the mixer on low speed, add the powdered sugar, 1/2 a cup at a time, until it is thoroughly mixed.
Add the vanilla and beat the frosting on medium speed for about 2 minutes.
Spread the frosting on the cooled cake.
Sprinkle the top of the cake with additional nuts if desired.
Store leftovers in an airtight container at room temperature for up to 24 hours and in the refrigerator for longer than 24 hours.
The calories shown are based on the cake being cut into 15 pieces and all of the frosting used. 1 serving is 1 slice of frosted cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.