Preheat the oven to 350°F (180°C). Lightly butter a 2-quart (2 liters) ceramic baking dish and place it inside a larger pan, you’ll use this later for the water bath.
Separate whites from yolks.
Add sugar over the egg yolks.
Mix until the mixture becomes pale and slightly thickened.
Add the lemon zest, lemon juice, melted butter, and vanilla extract, and whisk until fully combined.
Stir in the turmeric (if using) and flour, mixing just until the batter is smooth and lump-free.
Slowly pour in the milk, whisking as you go.
The batter will become very thin, this is exactly how it should be.
In a separate bowl, start beating the egg whites with a pinch of salt.
Mix until they form soft peaks.
Gently fold the egg whites into the batter in 2–3 additions.
Don’t overmix, the batter should look light, airy, and slightly lumpy.
Pour the batter into the prepared dish.
Carefully add hot water to the outer pan, filling it about halfway up the sides of the baking dish.
Bake for 35–40 minutes, until the top is lightly golden and set, but the center still has a slight jiggle.
Carefully remove the baking dish from the pan. Let the cake rest for 15–20 minutes before serving.
Dust with powdered sugar and serve warm.
Enjoy the cake. Serve as is, or with fresh berries on the side.