Preheat oven to 350°F (175°C).
Grease a 6–7" springform or cake pan. Line with parchment if needed.
Add cottage cheese, eggs, yogurt, sweetener, vanilla, zest, flour (optional), and protein powder to blender. Blend until completely smooth.
Taste and adjust for sweetness or lemon, if desired.
Pour into prepared pan and level out the top.
Bake for 40–50 minutes until edges are set and center slightly jiggles.
Turn off oven and crack door open for 10–15 minutes to cool gradually.
Let cake cool fully, then chill at least 2 hours (overnight best).
Slice, serve, and optionally top with berries, cinnamon, or nuts.