Sift flour and cornstarch with baking powder.
Whip butter with sugar until creamy and fluffy. Add egg yolks, one by one, while beating. Add sifted ingredients and lemon peel.
Place mixture on a horizontal surface and knead with hands, forming a ball. Cover with a cloth and set aside for 20 minutes.
If mixture seems too dry, add milk.
Flour the surface generously and roll dough to 3 mm/1/8 in. thick approximately. Cut circles (5 cm./2½ in).
Transfer them to a greased and floured cookie sheet.
Bake in preheated oven 350°F/175°C for 30 minutes or until edges begin to color. Remove from oven and allow to cool.
Take one cookie and generously spread with manjarblanco. Cover with another cookie.