Preheat oven to 180 degrees C. Line a large baking pan with parchment paper.

In a bowl combine flour, baking soda and salt. Set aside.

In a mixing bowl cream butter and peanut butter. Add brown sugar and beat until smooth. Add egg and vanilla and continue beating until well combined.

Gradually add dry ingredients to the mixture and whisk carefully, until a soft dough is formed.

Place white sugar on a plate. Make small bolls out of the dough and roll them in sugar to cover completely. Arrange them on the baking sheet, not too close one to another as they will increase their size during baking.

Bake the cookies for about 10 minutes or until they start getting brown around the edges. Remove from oven and let cool for 10 minutes before transferring them to a wire rack to cool completely.