Add this to the wet ingredients and stir on low speed with a stand mixer for 30 seconds or fold together with a spoon by hand. Stir in apples and sourdough starter discard by hand until mixed together.
Pour sourdough apple cake batter into the prepared pan. Bake for 35-40 minutes and until a toothpick inserted into the middle comes out clean. Let it cool before covering it with the maple cream cheese frosting.