In a medium size saucepan over medium-high heat, combine the butter, sugar, brown sugar, and milk. Whisk continuously until the butter has fully melted and then continue to whisk occasionally while brining the mixture to a boil.
Once boiling, allow mixture to continue boiling for another minute or so, then remove from heat and stir in the Biscoff Cookie Butter and vanilla extract. Mix well, until thoroughly combined.
Fold in the oats, then allow the mixture to sit for 5 minutes or so to thicken.
Then, using a 2 tablespoon cookie scoop, scoop cookies onto a lined baking sheet. Gently flatten the tops cookies by pressing down with the back side of a spoon (optional).
Place cookies in the refrigerator for about an hour to harden. These cookies tend to be pretty sticky, so a longer chill time is better.
Store leftover cookies in an airtight container in the refrigerator for up to 1 week or freeze for up to 3 months.
Notes
Using quick-cooking oats will yield a smoother, less chunky cookie.