Preheat the oven to 350°F (177ºC). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.

In a medium size bowl, combine the flour, crushed Biscoff cookies, cornstarch, baking soda, baking powder, and salt. Whisk together until everything is combined, then set aside.

In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and brown sugar on medium-high speed until light and fluffy, about 2-3 minutes. Add the egg and vanilla extract and beat again until everything is combined.

Add the cookie butter and mix until combined. Turn off the mixer, scrape down the sides and bottom of the bowl, then beat on low until everything is incorporated.

Turn the mixer speed to medium, then slowly add the flour and blend until no flour pockets remain. Cover the cookie dough and allow it to rest for 10 minutes.

Using a cookie scoop, drop rounded balls of dough onto the prepared baking sheet. Use your hands to roll them into a smooth ball. The dough will be soft but should not be sticky.

Bake the cookies for 8-10 minutes just until edges are just set. The centers will look underdone. Working quickly but carefully, use a large round cookie cutter or the rim of a glass or mug to reshape hot cookies. Allow to rest on the baking sheet for 10 minutes before transferring to a wire cooling rack to cool completely before making sandwiches.

In a small saucepan over medium heat or in a microwave safe bowl in the microwave, melt the butterscotch chips until smooth. Remove from heat and allow to cool for 10 minutes.

In a medium size bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine the cooled butterscotch mixture, butter, and marshmallow fluff, and beat on medium-high speed until smooth, about 2-3 minutes. Add water, starting with just 1 Tablespoon, and blend on medium speed until combined.

Turn the mixer down to low and slowly add the powdered sugar. Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. Add the salt, taste, and add more salt if needed. Turn off the mixer and use a spatula to stir the buttercream by hand to remove air bubbles and make it smooth and creamy.

When you are ready to assemble the sandwiches, pair the cookies, bottom to bottom, with like-sized/shaped cookies as best as possible.

Working with the bottom cookie of one pair at a time, add the frosting to a piping bag or use an offset spatula to spread frosting onto the bottom of the cookie. Leave about ½" border so you can "squish" the cookies together.

Take the top cookie and press the bottom of the cookie onto the fresh frosting to make a sandwich. Press the cookies together gently so the frosting moves to the edges. Cookies can be served immediately, but in my opinion, they get better as they sit! Store filled cookies covered tightly at room temperature for up to 5 days. Cookies freeze well, up to 3 months.