Preheat the oven to 350˚F. Line two baking sheets with parchment paper and set aside.
In a bowl, beat the butter, peanut butter, and brown sugar until light and fluffy.
Add the milk and vanilla to the wet mixture and beat until well combined.
Add the flour, baking soda and salt to the wet and mix until just combined.
For each cookie, scoop 1 teaspoon (6 g) of dough and roll it into a ball. Place the rounded scoops onto your baking sheet with enough room in between each for spreading. Use a fork to press a crisscross into each ball of dough.
Bake for 5-7 minutes, until lightly golden-brown. Allow to cool slightly then enjoy!