Preheat oven to 375 degrees F (190 degrees C).

Cream together the butter and peanut butter until smooth.

Gradually cream in the brown sugar until light and fluffy.

Beat in the egg until well combined.

In a separate bowl, sift together the flour and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Stir in vanilla if desired.

The dough will be very stiff.

Form the dough into long rolls about 1 inch in diameter.

Cut the rolls into 1-inch lengths and form them into balls.

Place the cookie dough balls about 1 inch apart on greased cookie sheets.

Press down the top of each cookie with a fork, first in one direction and then the other to make a crisscross design.

Bake in the preheated oven for 9 to 12 minutes, or until golden brown.

Remove the cookies from the pans while still slightly warm and cool on a wire rack.

Store in airtight containers.