Vanilla Cream Pie – The Best Romanian Cremsnit

eggs, separated

salt

g/ 10.6 oz/ 1 ½ cups granulated sugar

g/ 3.5 oz/ scant ¾ cups corn starch

vanilla extract, to taste

zest of ½ organic lemon, optional

liter/ 33.8 fl.oz/ 1 ¼ cups full-fat milk

g/ 7 oz butter

sheets of puff pastry, about 40x24 cm/ 15x10 inches (a bit more or less is OK)