Preheat the oven to 350°F and line a baking tray with parchment paper.
In a large mixing bowl, cream together the butter, Biscoff cookie butter, and brown sugar until smooth.
Mix in the egg and vanilla extract followed by the cinnamon, baking soda, and salt.
Stir in the flour until fully combined, then fold in the rolled oats.
Scoop the cookie dough onto the prepared pan in 1 tbsp measures and bake at 350°F for 10 minutes, or until the cookies are golden brown around the edges.
Transfer the cookies to a cooling rack and allow them to cool completely.
Spread a layer of cookie butter (or another filling) onto one cookie and sandwich it with another. Enjoy!