Pre-heat your oven to 375 degrees and grease a 9"x13" pan.
Add your peanut butter, softened butter, and brown sugar to a large mixing bowl and beat with a hand mixer just until combined. (If your butter is cold, you can just microwave the stick for 15 seconds.)
Add the egg, milk, and vanilla to the same bowl and beat again for a few seconds to combine.
Sprinkle the flour, baking soda, and salt on top of the peanut butter mixture and mix together with a spatula. (The dough will be thick.)
Fold in the chocolate chips.
Press the dough into the prepared baking dish. (I like to sprinkle a few extra chocolate chips on top.)
Bake for 20-25 minutes. The edges should be slightly browned.