Shape heaping tablespoons of dough into 21 balls. Using lightly floured hands, press thumb into center of each cookie to make deep indentation, but do not press all the way through. Place 1 dollop of the dulce de leche into center of each cookie, making sure to form dough around dollop to enclose. If necessary, work with lightly floured fingers so dough doesn’t stick. You may have to return dulce de leche dollops to freezer if they become too warm to work with.