In a medium sized mixing bowl, whisk together the flour, cocoa powder, salt, baking soda, and cornstarch. Set aside.
In a large mixing bowl. cream the butter until light and fluffy. Add in both sugars and mix on medium-high until completely incorporated.
Add in one egg at a time, beating well after each addition. Mix in the vanilla extract until combined.
Slowly mix in the dry ingredients with the butter/sugar mixture and beat until just combined. Mix in the milk until incorporated.
Fold in the chopped Biscoff cookies and chocolate chips. Combine into one large ball and wrap in press and seal. Put in the fridge to chill for at least 24 hours before baking.
Preheat oven to 350°. Prepare baking sheets with parchment paper. Remove cookie dough from fridge and let the cookie dough sit out at room temperature for about 30 minutes.
Take about ¼ cup of cookie dough and shape into larger balls. Place on prepared baking sheets (6 to a sheet). Put in the oven for 17-18 minutes. Edges will be golden brown in color. Cool on sheet.
Melt the ½ cup of cookie butter in a microwavable bowl for about 30 seconds. Drizzle over cooled cookies. Serve and enjoy!