1. 1. Prepare Your Baking Pan and Preheat the Oven: Start by preheating your oven to 350 F (175 C). Grease and flour a 9×13 inch baking pan, or line it with parchment paper for easy removal. This ensures your cake won’t stick and will come out cleanly.

2. 2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Make sure these are well combined so the leavening agent is evenly distributed throughout the cake batter. Set this bowl aside.

3. 3. Cream Butter and Sugar: In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar until the mixture is light and fluffy. This usually takes about 2-3 minutes and incorporates air, contributing to the cake’s tender texture.

4. 4. Add Eggs and Vanilla: Beat in the eggs one at a time, mixing well after each addition. Then, stir in the vanilla extract. Scrape down the sides of the bowl to ensure everything is fully incorporated.

5. 5. Alternate Wet and Dry Ingredients: Gradually add the dry ingredient mixture to the wet ingredients, alternating with the sourdough discard and milk. Begin and end with the dry ingredients. For example, add about one-third of the flour mixture, mix until just combined, then half of the sourdough discard, mix, then half of the milk, mix, and repeat. Be careful not to overmix the batter; mix only until the ingredients are just incorporated to keep the cake tender.

6. 6. Pour into Pan and Bake: Pour the finished cake batter evenly into your prepared 9×13 inch baking pan. Place it in the preheated oven and bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The cake should be golden brown on top and spring back lightly when touched.

7. 7. Cool Before Serving: Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for about 10-15 minutes. This allows the cake to set properly. After this initial cooling, you can carefully invert it onto the wire rack to cool completely before frosting or slicing.