In a large mixing bowl or the bowl of a stand mixer, mix the butter and sugars on medium-high speed until light and fluffy. Add the egg, vanilla bean paste and black coffee and mix until thoroughly combined, about 10 seconds more. Mixture may look slightly separated.

In a separate bowl, whisk the flour, cocoa powder, baking soda, and salt.

Add the dry ingredients to the butter and mix on low speed, scraping sides of bowl as necessary, until just barely combined, with no dry pockets remaining.

With a wooden spoon or flexible rubber spatula, fold in the chopped dark chocolate and the Milk Duds, reserving a few pieces of each to press into the tops of the cookies.

Cover and refrigerate until firm enough to scoop, at least 1 hour, or overnight.

Preheat oven to 350° F (175° C). Line a half sheet rimmed baking tray with parchment or a silicone mat.

With a cookie scoop, scoop chilled dough into 16 portions. Use your hands to roll each into portion a smooth ball. Arrange 8 balls on prepared tray, allowing plenty of space for the cookies to spread as they bake.

With the palm of your hand, flatten each ball slightly, then press a few additional Milk Duds and pieces of chocolate into the top of each cookie.

Freeze tray 10 minutes.

Bake in preheated oven for 11-14 minutes, or until puffy and set. Do not overbake.

For perfectly round cookies, quickly reshape with a round metal cutter.

Cool 10 minutes on tray, then transfer to a wire rack to cool completely.

Repeat with remaining balls of cookie dough.

Store Milk Dud cookies in an airtight container at room temperature for up to 5 days.