Stir together flour, sugar, salt, baking powder, baking soda, and cinnamon
Add carrots, oil, sourdough starter, and eggs
Mix until moistened, then beat at medium speed for 2 minutes.
Pour into greased and floured 13x19x2 inch baking dish
Bake at 325 degrees for approximately 55 minutes, until golden and toothpick comes out clean
While cake is baking, make cream cheese frosting. Mix together cream cheese, softened butter, powdered sugar, vanilla, and pinch of salt until smooth
Cool cake slightly and spread cream cheese frosting over top
Sprinkle with chopped nuts if desired