Preheat oven to 350. Lightly butter 2 baking sheets. Using electric mixer, beat 1 cup butter and sugar in large bowl until well blended. Beat in egg yolks, lemon extract and salt. Add flour in 2 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 or handle-end of a wooden spoon, make deep indentation in center of each ball. Fill indentations with scant 1/2 teaspoon jam or jelly.
Bake cookies until firm to touch and golden on bottom, about 22 minutes. Remove from oven. Transfer cookies to racks and cool completely. (Can be made 2 days ahead. Store between sheets of waxed paper in airtight container at room temperature. Cookies will soften slightly.)
Add miniature chocolate chips to dough and substitute almond extract for the lemon to make a linzer type cookie.