Preheat your oven to 350 degrees, and line two baking sheets with parchment paper. Set aside.
In a large bowl, beat together the peanut butter and sugar until just combined. Add in the milk and vanilla extract, and beat together until combined. It might look a little separated at this point- that's okay though.
Add in the flour, baking soda, and salt. Beat until a dough forms and there are no dry pockets of flour. Scoop the dough into rounded tablespoonfuls onto your prepared baking sheet about 2 inches apart.
Sprinkle the top of each cookie dough ball lightly with some extra sugar. Using a fork, press each dough ball down in one direction, and then in the opposite direction to create the cross-hatch look.
Bake for 12-13 minutes, or until the cookies are just becoming lightly brown on top. They will look puffy, but they will settle as they cool. Enjoy!