Preheat oven to 375 degrees. Using an electric mixer, cream the butter and sugar in a medium bowl. Beat in the peanut butter, egg yolk and vanilla. Stir in the flour and salt.
Form the dough into 1-inch balls. Roll them about 2 inches apart on a large cookie sheet. Bake the cookies for 5 minutes. Using a small melon baller or your thumb (but be careful not to burn yourself), make a deep indentation in the center of each cookie so that it will flatten slightly. With the melon baller, spoon about 1 teaspoon of the preserves into each cookie; do not overfill. Bake for about 15 minutes longer, until the cookies are golden and cracked. Transfer to a rack to cool completely. (The cookies can be stored for up to 3 days in an airtight container.)
Yield: 2 1/2 dozen cookies.