Preheat oven to 325 degrees
Line an 8x8 baking dish with parchment paper, leaving about two inches extra length on each side. Set the dish aside to prepare the batter.
In a large mixing bowl, or using a stand mixer, whisk the egg whites until they become thick and can hold a peak shape, then set the bowl aside.
In a medium bowl, combine the melted butter, sugar and the vanilla. Stir them together, and then add in the flour. Whisk together until it is smooth and creamy.
Whisk in the lemon juice and lemon zest, then add in the milk. Stir until they are well combined and become a thin batter.
Gently fold in the whisked egg whites, allowing some of the whites to float to the top. Do not over mix or the cake will be flat. It will look like shaving cream is in the cake batter, and that’s ok.
Pour the batter into the prepared baking dish and bake for 40-45 minutes, until the cake is golden brown on top.
Dust with powdered sugar before serving.