1. 1. Prepare Your Baking Pan and Oven: Begin by preheating your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan. If you prefer, you can 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, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed. This step is crucial for the cake’s rise and texture.

3. 3. Mix Wet Ingredients Separately: In another medium-sized bowl, combine the plain non-fat Greek yogurt, unsweetened applesauce, granulated erythritol (or your chosen sugar substitute), eggs, and vanilla extract. Whisk these ingredients together until they are smooth and well blended. If using, stir in the lemon zest here.

4. 4. Gently Combine Wet and Dry Mixtures: Pour the wet ingredient mixture into the dry ingredient mixture. Using a spatula or a wooden spoon, mix until just combined. Be careful not to overmix; a few small lumps are perfectly fine and will result in a more tender cake. Overmixing can lead to a tough texture.

5. 5. Pour into Prepared Pan and Bake: Scrape the batter into your prepared baking pan and spread it evenly with your spatula. Place the pan in the preheated oven.

6. 6. Check for Doneness: 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 lightly golden brown and spring back when gently touched.

7. 7. Cool Before Serving: 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 properly. Then, carefully invert it onto the wire rack to cool completely before slicing and serving.