Preheat your oven to 325°F (163°C). Lightly grease an 8x8 inch square baking pan, or line it with parchment paper for easy removal.
Carefully separate the egg yolks from the whites. Place the egg whites in a clean mixer bowl. Whip them using the whisk attachment until they form stiff, airy peaks. Transfer these whipped whites to a separate bowl and set aside.
In the mixer bowl, beat the egg yolks with the granulated sugar until the mixture is light and fluffy. Next, pour in the melted butter and vanilla extract, beating for another minute or two until everything is smoothly combined. Gradually add the all-purpose flour, mixing until the batter is completely smooth and lump-free.
Stir in the fresh lemon juice and vibrant lemon zest. Then, slowly pour in the lukewarm milk, bit by bit, beating gently until all ingredients are well blended.
Gently fold in about a third of your whipped egg whites into the batter using a rubber spatula. Repeat this process two more times with the remaining egg whites. Do not overmix; it's okay if a few white streaks remain.
Pour the light batter into your prepared baking pan. Bake for 40 to 70 minutes. The cake is ready when the top is lightly golden and feels firm to the touch, with a slight jiggle in the center.
Once baked, remove the cake from the oven and let it cool completely in the pan. This is crucial for the layers to set properly.
Before serving, dust generously with powdered sugar. Slice into squares and enjoy this refreshing summer treat!