Pre-heat oven to 350F (180C) and grease and flour an 8 inch square or round cake pan.
In a large bowl cream the butter and half the sugar (1/3 cup), add the zest and egg yolks beat to combine. Stir in (with a wooden spoon) the flour alternately with the milk. Then stir in the lemon juice. Combine well.
In another bowl beat the egg whites and remaining 1/3 cup sugar until stiff, then gently fold into the batter. Pour into the prepared cake pan. Place the cake pan in a larger high sided pan filled with 1 1/2 inches (4 cm) of hot water. Bake for approximately 25-30 minutes or until golden.
Let cool to warm, dust with powdered sugar and serve. Enjoy!
Notes
Store the leftovers tightly wrapped or in an airtight container in the fridge. It will keep for up to 3 days. Re-heating can be done in the microwave.