Mix sweetened condensed milk, 1 tin (300 ml) milk, egg yolks, corn starch and custard powder in a large microwave proof casserole dish. Whisk together until mixed well. Add the rest of the milk (5 tins or 1500 ml) and mix well again. Cook at high power in microwave for 3 minutes. Stir well and add margarine and vanilla. Cook for 3 minutes, stir well, continue cooking in two or three minute intevals until mixture is thickened and smooth. It takes about 8 minutes total in my microwave. Allow to cool slightly.
Beat reserved egg whites until stiffly beaten. Fold in egg whites into cooked milk mixture.
Pour into three baked pie crust - use Pat in Pan pie crust as a good pie crust for this pie.