Preheat your oven to 350°F (175°C). Grease a 9-inch round baking dish with butter and line with parchment paper.

In a large mixing bowl, whisk together the sugar and softened butter until light and fluffy, about 3-5 minutes. Add in the eggs, milk, and lemon zest; mix well.

Sift in the flour and gently fold into the mixture to create a smooth batter.

In a separate bowl, whisk together the egg yolks, cornstarch, and vanilla extract. Heat heavy cream until just simmering, then whisk into egg mixture.

Return mixture to heat and stir continuously until it thickens, about 5-7 minutes. Remove from heat and allow to cool slightly.

Fold the whipped egg whites into the batter and pour into the prepared baking dish. Layer the custard on top evenly.

Bake for 40-45 minutes until golden and a toothpick comes out clean. Cool in the pan for 10 minutes, then invert onto a wire rack.

Serve chilled or at room temperature.