Step 1: Preheat your oven to 350°F (175°C).
Step 2: Grease a 9-inch baking dish with melted butter or non-stick spray.
Step 3: In a large mixing bowl, whisk together the granulated sugar and flour until well combined.
Step 4: In a separate bowl, beat the egg yolks, then add melted butter, lemon zest, lemon juice, and milk. Mix until smooth.
Step 5: Pour the wet mixture into the dry ingredients and stir gently until just combined.
Step 6: In a clean mixing bowl, beat the egg whites until stiff peaks form.
Step 7: Carefully fold the whipped egg whites into the batter using a spatula.
Step 8: Pour the batter into the prepared baking dish and bake for 40-45 minutes.
Step 9: Allow the cake to cool for about 10 minutes before serving.