Preheat oven to 375°F (190°C).

Cream together the butter, peanut butter, brown sugar, sugar, egg, and vanilla until smooth.

In a separate bowl, whisk together the flour, baking soda, and salt.

Gradually blend the dry ingredients into the creamed mixture until just combined.

Shape the dough into 1-inch balls.

Roll each ball in sugar.

Place the sugared balls on an ungreased cookie sheet and flatten each one with a fork, making a crisscross pattern.

Bake for 10-12 minutes, or until the edges are lightly golden brown.

Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.