Heat the oven to 350 F. Place a rack in the center. Butter a 9 inch round cake pan, then line the base with parchment. Dust the sides with a little sugar if you like.
In a large bowl whisk the yogurt until smooth. Add eggs and whisk until the mix looks silky.
Sprinkle in cornstarch. Whisk until no dry spots remain. Add sugar and whisk again. Stir in vanilla and lemon zest if using.
Pour the batter into the pan. Tap the pan once on the counter to pop extra bubbles.
Bake until the top looks set and the center jiggles slightly, about 55 to 60 minutes. A toothpick near the edge should come out clean.
Cool the cake in the pan for 15 minutes. Run a thin knife around the edge. Invert onto a plate, then flip right side up.
Serve warm or at room temperature. Chill for a firmer slice. Add berries or a spoon of yogurt if you like.