To make the cake: Preheat the oven to 350°F. Lightly grease two 9" round layer pans, or one 9" x 13" pan., Beat together the oil, sugar, salt, eggs, spices, and baking soda., Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess.  , Add the flour, stirring until well blended., Add the carrots and nuts, and mix until just blended. Pour into the prepared pan(s)., Bake the cake(s) for 35 to 40 minutes, or until a cake tester inserted into the center comes out clean, or with a few moist crumbs clinging to it. Allow the cake(s) to cool completely before frosting. If you're using round layer pans, remove the layers from the pans after about 15 minutes, and place them on a rack to cool., To make the frosting: Beat the butter and cream cheese together until smooth. Add the salt and vanilla. Beat in the sugar. Add a teaspoon of milk or cream if the frosting is too stiff to spread; add additional sugar if it's too thin., Frost the sheet cake right in the pan. For the layers, frost the top of one layer, top with the second layer, and frost the top and sides of the cake. For helpful tips, see our blog post: How to assemble and frost a layer cake., Refrigerate the cake. An hour or so before serving, remove it from the refrigerator and bring it to room temperature. Refrigerate any leftovers, well wrapped, for a day or so; freeze for longer storage.