In a sturdy mixer, cream butter and peanut butter until smooth. Add sugars gradually and cream until fluffy. Add egg and vanilla and mix well.

In a separate bowl, sift together flour, baking soda, and salt. Add sifted dry ingredients gradually to butter mixture and mix until blended.

Using a round teaspoon of dough for each cookie, shape the dough into round balls. Roll the balls in the 1/3 cup granulated sugar to coat.

Use your thumb or a small kitchen implement to stamp small round indentations in each cookie (if the cookie cracks, it's OK - just don't go all the way through). Fill each hole with a tiny amount of blueberry jam.

Bake cookies at 350 degrees for 12 minutes, or until golden brown. Serves 8 to 10.