Use a handheld or standing mixer on medium speed to beat the butter until lighter in color and fluffy, about 20 seconds. Add the sugars and beat until creamed together on medium speed, for another 30 seconds. Beat in the egg and vanilla, scraping down the sides of the bowl as necessary.

In a separate bowl, mix together the flour, cocoa powder, baking soda, and salt. Over medium low speed, mix the dry ingredients into the wet ingredients. The dough will be very thick. Mix in the milk and chocolate chips by hand.

Cover the dough with saran wrap in a bowl and refrigerate for at least an hour, up to 36 hours.

After the dough has chilled, preheat your oven to 350 degrees F (180 degrees C). Line two baking sheets with parchment paper or silicone baking mats.

Roll the dough into balls a bit larger than a tablespoon each and arrange a few inches apart on the prepared baking sheets. You should have 16 cookies. Press them down just a bit and press pearls or sprinkles into the dough, if you’re using them. Bake for 12 minutes. Cookies will be soft and fudgy. Let cool for a few minutes on the baking sheets then remove to wire racks to cool completely.

In the meantime, using a handheld or standing mixer, whip together the peanut butter and powdered sugar until completely combined and fluffy, about a minute.

When cookies have cooled, spread about two tablespoons of the peanut butter mixture onto the bottom of a cookie and press another cookie on top of the peanut butter. Set the cookie aside and continue working until all cookies are sandwiched.