Preheat your oven to 325°F (160°C). Grease an 8x8 inch square baking pan.

Separate the egg yolks from the whites into two clean, dry bowls.

Melt the butter and let it cool slightly. Warm the milk until it is lukewarm (not hot).

In a large bowl, beat the egg yolks and sugar together until the mixture is pale and thick.

Whisk in the melted butter and vanilla extract.

Add the flour and salt, mixing until just combined.

Slowly pour in the lukewarm milk while whisking continuously.

Stir in the lemon juice and lemon zest. The batter will be very thin and liquidy—this is normal!

In a separate bowl, beat the egg whites (and cream of tartar if using) until stiff peaks form.

Gently fold the egg whites into the lemon batter in three additions. Do not over-mix; you want small clouds of egg white still visible on the surface.

Pour the batter into the prepared baking pan.

Bake for 40-50 minutes, or until the top is golden brown and the cake has a slight jiggle in the center.

Let the cake cool completely in the pan at room temperature, then refrigerate for at least 2 hours before slicing.

Dust the top generously with powdered sugar.

Slice into squares and serve with fresh berries and whipped cream.