Preheat convection oven to 165°C (330°F) and line a 7-inch round cake pan with parchment paper.
In a large mixing bowl, whisk eggs and erythritol for 3 minutes until the volume increases and the mixture is pale.
Gently fold in the Greek yogurt and vanilla extract until the mixture is homogeneous.
Sift the whey protein, coconut flour, cornstarch, and salt into the wet ingredients.
Whisk until smooth, ensuring no dry clumps of flour or protein remain, then fold in lemon zest.
Transfer the batter to the prepared cake pan and smooth the top with a spatula.
Bake for 45 minutes or until the cake is set and a toothpick inserted into the center comes out clean.
Remove from oven and allow to cool at room temperature in the pan for 30 minutes.
Refrigerate for at least 2 hours before slicing to allow the protein and fiber structure to fully set.