Preheat the oven to 350 degrees F (175 degrees C). Lightly grease an 8-inch square baking dish.

Beat egg yolks, lemon juice, butter, and lemon zest together in a large bowl with an electric mixer until thick and lemon colored.

Combine sugar, flour, and salt; add alternately with milk to the lemon mixture, beating well after each addition.

Beat egg whites in a glass, metal, or ceramic bowl until stiff.

Gently blend egg whites into batter on low speed with an electric mixer. Pour into prepared baking dish.

Place a pan of hot water in the preheated oven; set the baking dish into the pan; bake for 45 minutes. For best results, serve warm.