Using a large bowl, fold the confectioners' sugar a little at a time into the stiffly beaten egg whites. Add the crackers, nuts, and vanilla, and fold in gently. Melt the chocolate over a double boiler and allow to cool slightly. Fold the melted chocolate into the egg white mixture. Using a greased cookie sheet, drop the mixture by rounded half teaspoonfuls. Place in the oven and bake for 12 minutes. Remove cookies from oven and repeat until batter has been used up.