1. Preheat and Prepare: Start by preheating your oven to 350 F (175 C). Grease and flour a 9-inch round cake pan or an 8×8 inch square baking pan. You can also line the bottom with parchment paper for easier removal.

2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed. This ensures the leavening agents are spread throughout the cake for an even rise.

3. Mix Wet Ingredients: In a large bowl, whisk together the granulated sugar and melted butter until well combined. Then, beat in the large egg and vanilla extract until the mixture is smooth and light.

4. Incorporate Yogurt and Sourdough Discard: To the wet ingredients, add the plain whole milk yogurt and the sourdough discard. Whisk everything together until just combined. Don’t overmix; a few small lumps are perfectly fine at this stage.

5. Combine Wet and Dry: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or wooden spoon until just combined. Again, avoid overmixing. Overmixing can develop the gluten in the flour too much, leading to a tougher cake. Mix until no streaks of dry flour remain.

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

7. Cool and Serve: 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 before you attempt to remove it. After cooling slightly, invert the cake onto the wire rack to cool completely before slicing and serving.