Line a 9 x 5 inch loaf with parchment or grease well.
Crumb Topping
In a small bowl combine all topping ingredients and use a fork or your fingers to combine until crumbly. Set aside.
Cookie Layer
In a bowl mix together softened butter and sugars with an electric hand mixer or by hand until combined and light and fluffy.
Add in egg and vanilla extract. Mix to combine.
Add in dry ingredients and mix just until combined.
Press cookie layer into bottom of prepared 9 x 5" loaf pan and bake for 12 minutes.
Put Biscoff cookie butter into a microwave safe bowl and microwave for 30 seconds or until melted.
Remove bars from oven and drizzle with half of Biscoff cookie butter.
Add marshmallows and then sprinkle crumb topping over cookie layer.
Bake for 12-15 more minutes or until marshmallows are golden brown.
Remove from oven and drizzle with remaining Biscoff.
Allow to cool in pan for 10 minutes and remove from pan.
Notes
For the best texture, be careful not to overbake your Biscoff bars. You want the cookie base to stay nice and soft and chewy. The bars will continue to set up as they cool, so it’s okay if the center looks slightly soft when you take them out of the oven.
Lining your loaf pan with parchment paper makes it easy to lift the bars out of the pan once they have cooled. You can find my parchment sling tutorial here.
Warm the cookie butter for just a few seconds in the microwave before adding your final drizzle. You want it thin enough to drizzle easily but not hot.
Allow the bars to cool before slicing. The cookie butter and marshmallow layers need a little time to set, and you’ll get much cleaner slices if you can resist cutting into them right away! But they are gooey and delicious eaten warm!