Preheat oven to 350°F if baking immediately. Grease a 9x13-inch cake pan.
In a large bowl, whisk together oil, sourdough starter, eggs, vanilla, and sugars.
In another bowl mix dry ingredients: flour, baking soda, baking powder, salt, and cinnamon.
Add the dry mix into the wet until just incorporated.
Stir in carrots and walnuts until just combined.
If you are wanting to ferment batter, cover with plastic wrap and refrigerate for several hours or overnight.
Pour batter into prepared pan and bake for approximately 45 minutes, testing with a toothpick at 40 minutes to see if it comes out clean. Do not over bake.
Set aside to cool completely.
Frosting:
With a mixer, whip cream cheese and butter until smooth.
Add icing sugar and vanilla, mixing well.
Frost cake and add chopped nuts if desired.
Notes
Make sure and store frosted cake in the fridge.
If fridge space is at a premium, frost just before serving.