1. Preheat your oven to 350 F (175 C). 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 powder, baking soda, and salt. Make sure there are no lumps, and set aside.

3. In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter and packed light brown sugar until the mixture is light and fluffy. This usually takes about 2-3 minutes. The color will lighten, and the texture will become airy.

4. Beat in the large egg until fully incorporated, then stir in the vanilla extract. Next, add the sourdough discard and mix until just combined. Don’t overmix at this stage; just ensure everything is blended.

5. Gradually add the dry ingredient mixture to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients. For example, add about one-third of the flour mixture, mix until just combined, then add half of the buttermilk, mix, then another third of the flour, the remaining buttermilk, and finally the last third of the flour. Mix until just combined after each addition, being careful not to overmix the batter.

6. Pour the batter evenly into your prepared 8×8 inch baking pan. Spread it gently with a spatula to ensure an even layer.

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 touched.

8. Let the cake cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving. This allows it to set properly and prevents it from crumbling.