1. 1. Prepare Your Baking Pan and Oven: Begin by preheating your oven to 350 F (175 C). Grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan. You can also line the bottom with parchment paper for easy removal. This ensures your cake won’t stick.

2. 2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt. Make sure there are no lumps and the spices are evenly distributed. Set this bowl aside.

3. 3. Cream Wet Ingredients: In a large mixing bowl, using an electric mixer (handheld or stand mixer), cream together the softened unsalted butter, granulated sugar, and light brown sugar until the mixture is light and fluffy. This usually takes about 2-3 minutes. Scrape down the sides of the bowl as needed.

4. 4. Add Egg and Flavorings: Beat in the large egg until it’s fully incorporated, then stir in the vanilla extract. Continue mixing until the mixture is smooth and well combined.

5. 5. Incorporate Sourdough Discard and Maple Syrup: Gradually mix in the room temperature sourdough discard and pure maple syrup until just combined. The mixture might look a little curdled at this stage, but don’t worry, it will come together.

6. 6. Alternate Dry and Wet Ingredients: Add about one-third of the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Then, pour in half of the milk and mix until incorporated. Repeat this process, adding another third of the dry ingredients, then the remaining milk, and finally the last of the dry ingredients. Mix only until no streaks of flour remain; overmixing can lead to a tough cake.

7. 7. Fold in the Diced Apple: Gently fold in the finely diced apple using a spatula. Distribute the apple pieces evenly throughout the batter.

8. 8. Bake the Cake: Pour the batter into your prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should be golden brown and spring back lightly when touched.

9. 9. Cool and Serve: Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for at least 10 minutes before attempting to invert it or slice. This allows the cake to set and prevents it from crumbling.