Yeast potato biscuits

The yeast dissolves in a small amount of sweetened milk, mixes with a tablespoon of flour and is left to rise. The potatoes boiled the day before, peeled and grated, are mixed with the remaining milk, sourdough, eggs, salt, sugar, butter (from which one spoon is left to lubricate cookies), lemon zest, cinnamon and flour. The dough is thoroughly kneaded and left for 1 hour to rise. Then it is put in a small, well-oiled form. In front of the oven, the biscuits are spread over the top with the remaining butter. It bakes for about 45 minutes until it takes on a golden hue. Cookies are served to the table with sprinkled vanilla sugar.