Preheat oven to 375°F. Lightly grease 2 baking sheets.
Combine flour, baking soda and salt together in a bowl and set aside.
In the bowl of an electric mixer, combine butter, peanut butter, sugar, brown sugar, egg and vanilla. Slowly add in flour mixture and mix until fully incorporated.
Form dough into balls, place on a greased baking sheet and smash down with a fork twice to make an X.
For crunchy cookies bake until golden brown, about 9 minutes. For soft cookies bake 5 minutes. Let cool on baking sheets for a couple minutes before removing.