• Preheat oven to 350°F. Lightly butter 2 baking sheets. Using electric mixer, beat butter and sugar in large bowl until well blended. Beat in yolks, lemon peel, lemon juice and salt. Add flour in 23 additions and beat just until moist clumps form. Gather dough together in bowl to bind dough. Form dough into 1-inch balls. Place balls on prepared baking sheets, spacing 1-inch apart. Using finger, make deep indentation in center of each ball.
• Bake cookies until firm to touch and golden on bottom, about 22 minutes. Remove from oven. Immediately fill indentation in each cookie with scant ½ teaspoon jam. Transfer cookies to racks and cool completely.

Do Ahead: Can be made 2 days ahead. Store between sheets of waxed paper in airtight container at room temperature. Cookies will soften slightly.