Preheat the oven to 350°F (175°C). Grease and flour the cake pans.

In a large mixing bowl, whisk together the sourdough starter, granulated sugar, brown sugar, and vegetable oil until well combined.

Add the eggs one at a time, mixing well after each addition.

Stir in the grated carrots and crushed pineapple.

In another bowl, combine the flour, baking soda, baking powder, cinnamon, nutmeg, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the walnuts if using.

Divide the batter evenly between the prepared cake pans.

Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

In a mixing bowl, beat the cream cheese and butter together until smooth and creamy.

Gradually add the powdered sugar and vanilla extract, mixing until well combined and fluffy.

Once the cakes are completely cooled, frost the top of one layer, place the second layer on top, and frost the top and sides of the cake.