Preheat your oven to 375°F (190°C).

In a stand mixer, cream together the softened butter, peanut butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg, milk, and vanilla extract to the butter mixture and mix until combined.

In a separate bowl, sift together the flour, baking soda, and salt.

Gradually add the flour mixture to the wet mixture on low speed, mixing until just combined.

Scoop one-inch balls of dough, rolling them into tight balls.

Roll each dough ball in extra granulated sugar and place them on a cookie sheet about 2 inches apart.

Using the back of a fork, gently press each dough ball to flatten and create a hashtag pattern.

Bake the cookies in the preheated oven for 8 to 10 minutes, until the bottoms are lightly golden brown.

Allow the cookies to cool on the baking sheet for a couple of minutes before transferring them to a wire rack to cool completely.