Preheat oven to 350°F. Line a baking sheet with parchment paper.

Place peanut butter, butter and brown sugar in a large bowl. Cream together using a hand-held mixer until soft and fluffy. Alternatively, you can do this in a stand-mixer fitted with the paddle attachment.

Add egg yolk and vanilla and mix again until incorporated.

Whisk together the flour, baking soda and salt in another bowl.

Add the dry ingredients to the peanut butter mixture and stir together with a spatula until no more streaks of flour can be seen in the dough.

Add the milk and continue mixing until incorporated.

Using a small or medium cookie scoop, scoop the batter into the palm of your hand and roll into a ball shape. Place on the prepared baking sheet.

Repeat with remaining dough.

Using the back of a fork, gently press down on each cookie then turn the fork at a 90° angle and press again to create a cross-hatch pattern.

Repeat with all the cookies.

Bake for 12-15 minutes until the edges are just starting to turn light brown. Cookies will still be soft when taken out of the oven. Let them cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.