Preheat oven to 350F. Line two baking sheets with a silicone reusable baking sheet or parchment paper.
In a small mixing bowl, combine the flour, baking soda, baking powder, and salt. Whisk to aerate and combine all ingredients. Set aside.
In the bowl of a stand-mixer fitted, beat the butter and peanut butter together until smooth and well incorporated.
Add the sugars (brown and granulated) to the butter mixture and mix for 2-3 minutes until the mixture appears light and fluffy.
Add the egg and mix until combined. Then mix in the milk and vanilla.
Slowly add the flour mixture and beat until thoroughly combined.
Using your hand or a medium cookie scoop, portion out approximately 2 tbsp of dough. Place on baking sheet, allowing room for the cookies to spread out while baking (2 inches between cookies).
Bake for 10-14 minutes (mine were perfect at 13 min) being careful not to over bake. You know when these cookies are done as they will begin to crack on top and the edges will be slightly firm to the touch. The cookies will continue to bake once they are out of the oven, continuing to crack on the tops.
Keep your cookies on their cookie sheet for 5 minutes to cool. Then, once firm enough to lift, transfer them to a wire rack.