Heat oven to 350°F.
In large bowl, break up cookie dough. Stir or knead in flour until well blended. Shape dough into 40 balls. Place egg whites in shallow bowl. Place chopped walnuts in another shallow bowl. Dip each ball into egg whites, then roll each ball in chopped walnuts. Place cookies 2 inches apart on ungreased cookie sheets. Press thumb in center of each cookie to make indentation.
Bake 10 to 14 minutes or until cookies are set around edges. Remove cookies to cooling rack to cool completely, about 30 minutes. Place 1 teaspoon dulce de leche in indentation of each cookie.
In small microwavable bowl, microwave chocolate uncovered on High 30 seconds; stir. Continue to microwave in 15-second increments until chocolate is melted and smooth. Drizzle chocolate mixture over cookies.