Prepare a baking form with a releasable bottom. You have to wrap the inside and all of the outside of the form with aluminum foil (about 2-3 layers of foil).
Beat the butter with cream cheese (both at room temperature) and gradually add the lemon juice, the egg yolks one at a time, milk and the mixed and sifted together flour and starch.
Stir the mixture with the egg whites that you've beaten with the salt and sugar. Pour the mixture into the prepared baking form and put it to bake in a water bath with pre-boiled water. The water should reach halfway up the baking form. Bake in preheated oven at 320°F (160 °C) for about 1 hour. Do not open the oven door during that time.
Take the baked Japanese cheesecake out and carefully release it from the baking form and baking paper. Cool and serve it with jam or powdered sugar.