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!