Prepare the Batter:

Preheat your oven to 325°F (160°C). Grease and line an 8×8-inch baking pan with parchment paper.
In a mixing bowl, beat the egg yolks and sugar until pale and creamy. Stir in the melted butter, lemon zest, and lemon juice, mixing until smooth.
Sift in the flour and mix until just combined.
Add Milk and Egg Whites:

Slowly add lukewarm milk to the batter, stirring continuously until smooth. The batter will be thin.
In a separate bowl, beat egg whites until stiff peaks form. Gently fold the egg whites into the batter, a third at a time, until just incorporated. The batter will look lumpy, but that’s okay!
Bake the Cake:

Pour the batter into the prepared pan and bake for 40-50 minutes, or until the top is golden and the center is set.
Let the cake cool in the pan before removing and slicing.
Serve:

Dust with powdered sugar before serving. Enjoy chilled or at room temperature.