Preheat the oven to 375°F (190°C). A fully heated oven helps the cookies bake evenly and hold their shape.

In a large bowl, mix the butter, peanut butter, brown sugar, milk, and vanilla extract at medium speed until smooth and creamy.

Add the egg and continue mixing until fully incorporated. If the mixture looks slightly separated at first, keep mixing until smooth.

Add the flour, baking soda, and salt. Mix on low speed until just combined. If the dough feels too soft, let it rest for a few minutes or add a tablespoon of flour.

Scoop and roll the dough into small balls, place them on a baking tray, and gently flatten with a fork to create a crisscross pattern.

Bake for 7–8 minutes until the edges are lightly golden while the centers remain soft.

Let the cookies rest on the tray for 2 minutes, then transfer to a cooling rack to cool completely.