1. Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan, or line it with parchment paper, leaving an overhang on two sides to easily lift the cake out later.

2. In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set this aside for now.

3. In a large mixing bowl, using an electric mixer (handheld or stand mixer), cream together the softened butter, granulated sugar, and light brown sugar until the mixture is light and fluffy, about 2-3 minutes. The mixture should look pale and airy.

4. Beat in the egg until well combined, then stir in the vanilla extract and sourdough discard. The mixture might look a little curdled at this point, but don’t worry, it will come together. Gradually add the milk, mixing until just combined.

5. Gradually add the dry flour mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; a few streaks of flour are fine. Overmixing can lead to a tough cake.

6. Gently fold in the diced apple and ground cinnamon until evenly distributed throughout the batter. If using, fold in the chopped walnuts or pecans as well.

7. Pour the batter into your prepared 8×8 inch 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 when lightly touched.

8. While the cake cools slightly in the pan (about 10 minutes), whisk together the powdered sugar, 2-3 tablespoons of milk, and 1/2 teaspoon of ground cinnamon in a small bowl until smooth. If the glaze is too thick, add a tiny bit more milk; if too thin, add a little more powdered sugar. Once the cake has cooled for a bit, drizzle the glaze generously over the top. Let the glaze set before slicing and serving.