Grate the carrots and set aside.
In a mixer, combine sugar, oil, and eggs. Beat until creamy. Add baking powder, salt, and cinnamon. Gradually add the flour. Beat until smooth. Remove from mixer and fold in grated carrots with a spoon (and walnuts if desired). Pour batter into a 9 x 13 x 2 greased and floured cake pan. Bake at 350 degrees for 35-40 minutes. Test for doneness with a toothpick.
1/2 cup butter (room temperature), 1 - 8oz. package Philly cream cheese, room temperature (or from refrigerator, microwave :20 seconds on high--must remove cream cheese from foil before putting in microwave), 2 cups powdered sugar , 1 tablespoon lemon juice.
Cream together the butter and cream cheese until smooth. Slowly whip in the powdered sugar and lemon juice. Ice the cake when cake is cool.