Biscoff Cookie Butter Cookies With Butterscotch Frosting

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 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 (I use this #50 cookie scoop for all of my standard size cookies), 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. Remove from the oven and allow to cool on the baking sheet for 10 minutes before transferring to a wire cooling rack to cool completely before frosting.

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 15 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 and the butter and beat on medium-high speed until smooth, about 2-3 minutes. Add the vanilla extract and 2 Tablespoons of the milk or cream 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. This usually takes about 3-5 minutes.
When your buttercream is smooth, use a small offset spatula to spread frosting onto each cookie. Sprinkle additional crushed Biscoff crumbs³ on top of frosting before it sets, if desired. Store leftovers covered tightly at room temperature up to 5 days. Cookies freeze well, up to 3 months. Thaw at room temperature.