Preheat the oven to 325 degrees Fahrenheit and line a 9x13 inch baking pan with parchment paper then spray the sides with non-stick cooking spray.
Separate the eggs by placing the whites in a large bowl and the yolks into another large bowl.
Add the cream of tartar to the egg whites then whip with an electric mixer until stiff peaks form, about 3 minutes then set aside.
Whip the egg yolks, sugar and vanilla together until the eggs turn a pale-yellow color, about 1 minute.
Stir in the milk, butter, lemon juice and lemon zest until combined then stir in the flour.
Fold in the whipped egg whites then pour the mix into the prepared pan and bake for 40-45 minutes or until the center jiggles only slightly when moved.
Let it cool for at least 30 minutes before dusting the top with powdered sugar and serving.