1. 1. Prepare Your Baking Pan and 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. This ensures your coffee cake won’t stick and will release easily after baking.

2. 2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Make sure these are well combined to ensure even leavening throughout the cake. Set this bowl aside for a moment.

3. 3. Cream Wet Ingredients: In a large mixing bowl, using an electric mixer or a whisk, cream together the softened unsalted butter and 3/4 cup granulated sugar until the mixture is light and fluffy. This step incorporates air, contributing to the cake’s tender texture.

4. 4. Add Egg, Vanilla, Sourdough Discard, and Milk: Beat in the large egg and vanilla extract until fully incorporated. Then, add the sourdough discard and milk to the butter mixture, mixing until just combined. Don’t worry if it looks a little curdled at this stage; it will smooth out.

5. 5. Combine Wet and Dry Mixtures: Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix; overmixing can lead to a tough cake. A few lumps are perfectly fine.

6. 6. Create the Chocolate Swirl Mixture: In a small bowl, combine the 1/2 cup chocolate chips, 2 tablespoons granulated sugar, and 1 tablespoon unsweetened cocoa powder. Stir them together until the chocolate chips are coated. This simple mixture will create your beautiful swirl.

7. 7. Assemble and Swirl: Pour about two-thirds of the cake batter into your prepared baking pan and spread it evenly. Sprinkle about half of the chocolate swirl mixture over this layer. Dollop the remaining cake batter over the chocolate layer, then sprinkle the rest of the chocolate swirl mixture on top. Use a knife or a skewer to gently swirl the chocolate mixture into the batter, creating a marbled effect. Don’t overdo the swirling, or the chocolate will blend in too much.

8. 8. Bake and Cool: Bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should be golden brown. Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for at least 10-15 minutes before slicing and serving. This allows the cake to set properly.