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 for easy removal.

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

3. In a large mixing bowl, using an electric mixer or a sturdy whisk, cream together the softened unsalted butter and packed light brown sugar until the mixture is light, fluffy, and pale in color (about 2-3 minutes).

4. Beat in the large egg until fully incorporated. Then, add the sourdough discard and vanilla extract, mixing until just combined. Do not overmix.

5. Gradually add the dry ingredient mixture to the wet ingredients, alternating with the milk. Start and end with the dry ingredients. Mix on low speed or by hand until just combined. Be careful not to overmix the batter; stop as soon as no dry streaks of flour remain.

6. Pour the batter evenly into your prepared 8×8 inch baking pan. Smooth the top with a spatula.

7. Bake for 20-25 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The cake should be golden brown around the edges and spring back lightly when gently pressed.

8. Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for at least 15 minutes before slicing and serving.