Preheat oven to 325°F (160°C). Grease an 8x8-inch baking dish.
In a mixing bowl, beat the egg whites until stiff peaks form, then set aside.
In another bowl, whisk egg yolks, sugar, melted butter, vanilla extract, and lemon juice until smooth.
Gradually add flour and mix well. Slowly incorporate the lukewarm milk while stirring until combined.
Gently fold in the beaten egg whites until just mixed.
Pour the batter into the prepared baking dish and bake for 40-45 minutes until the top is golden brown.
Let the cake cool completely, then dust with powdered sugar before serving.