Whisk together flour, baking soda and salt in a medium sized bowl.

Using a stand mixer fitted with the paddle attachment or a hand mixer, cream together butter and peanut butter until well combined.

Add brown sugar and granulated sugar and cream until light and fluffy, about 2 minutes.

Add in egg, vanilla and milk and mix well.

Add in dry ingredients and mix until just combined, scraping down the sides and bottom of the bowl.

Chill the dough in the refrigerator for at least 30 minutes.

While the dough is chilling, preheat the oven to 375 degrees and line baking sheets with parchment paper.

Scoop out dough into one inch balls, roll gently with your hands and roll in granulated sugar.

Place dough balls several inches apart on the baking sheet and use a fork to gently press a criss cross hash mark into each cookie, flattening slightly.

Bake for 10 minutes, rotating baking sheet halfway through.

Allow to cool on baking sheet for 1-2 minutes then transfer to a wire rack to cool completely.