• Preheat the oven to 350ºF (180ºC).
• Prepare 3 (9-inch) round cake pans by spraying them with cake release spray (or lightly oiling them) and lining the bottom with a piece of parchment paper cut to size.
• Add the unfed sourdough starter, oil, and sugar to a large bowl. Use an electric mixer or stand mixer with the whisk attachment to mix well. It’s ready for the next step when the starter is fully incorporated into the mixture and the sugar is beginning to dissolve.
• Add the eggs one at a time, mixing until the egg is fully incorporated between each addition. Add in the vanilla extract and mix again.
• Add the crushed pineapple and carrots to the wet ingredients. Using a spatula, fold them into the mixture until evenly distributed. Set aside.
• In a separate medium-sized bowl, add the dry ingredients. This includes the flour, cinnamon, allspice, nutmeg, salt, and baking soda. Whisk well to combine.
• Add the dry ingredients to the wet ingredients and gently stir just until combined. Do not over-mix. Some lumps in the batter are okay, however you want to make sure there is no flour mixture on the bottom of the bowl as this can affect how the cake bakes. (dry crumb on top of the layer)
• Pour the cake batter into each pan, adding a little at a time to ensure it is evenly distributed.
• Bake the cakes for 25-30 minutes, or until a toothpick comes out clean in the center and the sides of the cake are starting to pull away from the sides of the pan.
• Remove from the oven and allow the cakes to cool in the pan for 25 minutes.
• Carefully remove the cakes from the pans and allow them to cool completely on a wire rack before frosting.
• If you’re preparing the cake the day before, you can store the layers in the fridge overnight. Wrap each cake layer in plastic wrap and transfer to the fridge. This will help to prevent the cake layers from drying out.

Prepare the frosting

• Add the softened butter and cream cheese to the bowl of a stand mixer. Using the whisk attachment, cream the two together until smooth and fluffy, scraping down the sides of the bowl as needed. (Note: If you don’t have a stand mixer you can use a large bowl and an electric hand mixer).
• Whisk in the vanilla extract and salt.
• Slowly add in the powdered sugar by adding in about ½ cup at a time, making sure the sugar is fully incorporated after each addition. The frosting should be thicker than most cream cheese frostings, but not stiff.
• If the frosting is too thick, slowly add some milk, using about 1 tablespoon at a time until you get your desired consistency. I don’t typically need to add milk, but it’s a great tool to have in your back pocket if the frosting turns out stiff.

Assemble the cake

• Before you get started: If the cakes have a domed appearance on top, you will need to shave off the “dome” until the cake appears flat. I typically do not have that issue with sourdough cakes, but it can happen.
• To assemble the cake, place 1 cake layer on a cake round or a cake stand, top side down.
• Add a dollop of frosting and spread an even layer of frosting that reaches the edge of the cake and is about ½” thick. Place another cake layer on top and repeat the process until each cake layer is frosted.
• Finally, frost the edges of the cake.
• Once the cake is fully frosted, sprinkle the edges with some chopped walnuts (if desired).
• Slice, serve and enjoy! When not in use, be sure to wrap it tightly and store in the refrigerator. Do not store your carrot cake at room temperature.