Preheat the oven to 325°F (160°C). Line an 8×8 inch baking pan with parchment paper or spray it with non-stick spray. Keep mixing bowls ready.

In a clean stand mixer or bowl, beat egg whites until stiff peaks form. Set aside. This provides your cake with a light sponge finish top.

In another bowl, the egg yolks and sugar are whisked until pale and creamy. The melted butter and vanilla extract are added and combined until smooth.

Sift in the flour and gently whisk. Introduce the lemon juice and zest; stir to combine—a bit lumpy is OK.

A little bit of milk is introduced at a time while gently stirring. The batter is going to be very thin—that’s exactly what makes for the layers.

Gently fold in the egg whites in three batches with a spatula. It will not be completely smooth-soft clumps are fine.

Pour the batter into the prepared baking pan. Put in an oven for 50-55 minutes until the top set, golden in color. The center still jiggles slightly. Let it cool completely before slicing.

Once the cake is cool, snow-dust it with powdered sugar, cut into perfect squares, and serve.