1. Preheat 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.

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

3. In a large mixing bowl, using an electric mixer, cream together the softened butter and granulated sugar until the mixture is light and fluffy, about 2-3 minutes.

4. Beat in the eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed. Stir in the vanilla extract until just combined.

5. With the mixer on low speed, gradually add about one-third of the dry ingredient mixture to the wet ingredients, mixing until just combined. Then, add half of the sourdough discard, mixing until incorporated. Repeat with another third of the dry ingredients, followed by the remaining sourdough discard. Finish by adding the last third of the dry ingredients, mixing only until no streaks of flour remain. Be careful not to overmix.

6. Pour the batter evenly into your prepared 9×13 inch baking pan. Spread it gently with a spatula to ensure an even layer. Bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The cake should be golden brown around the edges.

7. Remove the cake from the oven and let it cool in the pan on a wire rack for at least 30 minutes before attempting to frost it.

8. To prepare the optional frosting: While the cake cools, in a medium bowl, beat the optional softened butter until creamy. Gradually add the optional powdered sugar, beating until smooth. Stir in the optional vanilla extract, then add optional milk or cream one tablespoon at a time until you reach your desired consistency. Spread the frosting evenly over the cooled cake.