1. 1. Prepare Your Baking Pan and Preheat Oven: Start by preheating your oven to 350 F (175 C). Grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan. You can also line the bottom with parchment paper for easier removal. This ensures your cake won’t stick.

2. 2. Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking soda, and salt. Make sure there are no lumps in the cocoa powder, creating a uniform dry mixture.

3. 3. Mix Wet Ingredients: In a separate medium bowl, whisk together the sourdough discard, milk, vegetable oil, vanilla extract, and the large egg until well combined and smooth. The discard might have a few small lumps, which is fine.

4. 4. Combine Wet and Dry Mixtures: Pour the wet ingredient mixture into the dry ingredient mixture. Stir with a whisk or a spatula until just combined. Be careful not to overmix; a few small lumps are okay. Overmixing can lead to a tough cake.

5. 5. Add Hot Liquid: Carefully pour the hot water (or hot coffee) into the batter. Stir gently until the batter is smooth and pourable. The hot liquid helps to bloom the cocoa powder, intensifying the chocolate flavor and creating a tender crumb. The batter will be quite thin.

6. 6. Pour and Bake: Pour the thin batter evenly into your prepared baking pan. Place the pan in the preheated oven and bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should spring back lightly when gently pressed.

7. 7. Cool the Cake: 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 and prevents it from breaking when you remove it. After 10-15 minutes, carefully invert the cake onto the wire rack to cool completely before frosting or serving.