Lightly grease a cookie sheet with butter. Combine in a medium mixing bowl 1 cup smooth peanut butter, brown sugar, baking soda, and egg. Roll into 12 balls and evenly space on the cookie sheet. Slightly press cookies down and make a small indentation with index finger. Place 1 teaspoon jam in the indentation. Bake cookies for 15 minutes. Let cool on the pan for 5 minutes and transfer to a wire baking rack to cool completely.