Beat butter at medium speed with an electric mixer until creamy; gradually add 2/3 cup sugar, beating well. Add egg yolks and vanilla, beating until blended.
Combine flour and salt; add to butter mixture, beating at low speed until blended. Cover and chill dough 8 hours.
Shape dough into 1" balls, and place 2" apart on ungreased baking sheets. Press thumb into each cookie to make an indentation.
Bake at 350° for 12 to 13 minutes. Cool 1 minute on baking sheets; remove to wire racks. Make thumbprint indentations again while cookies are still warm; let cookies cool completely.
Meanwhile, combine 1/3 cup powdered sugar and heavy cream in a small bowl; stir with a fork until smooth. Spoon icing into a zip-top freezer bag; cut a tiny hole in 1 corner of bag, and squirt a small amount of icing into indentation in each cookie. Press jelly beans into center of each cookie. Let set. Dust cookies with powdered sugar.