With an electric mixer, cream the butter and sugar in a medium bowl. Beat in
the peanut butter, egg yolk and vanilla extract. Stir in the flour and salt.
Form the dough into 1-inch balls. Roll them in peanuts and place 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.
for about 15 minutes longer, until the cookies are golden and cracked.
to a rack to cool completely.
The cookies can be stored for up to 3 days in an airtight container.