Line a 9x13-inch baking dish with parchment paper. Lightly grease the paper with butter or cooking spray. This helps the treats lift out easily later and keeps the sticky mixture from sticking.

Place a large pot over low heat. Add the butter and let it melt slowly. Stir gently while melting so the butter heats evenly without burning or turning brown.

Add mini marshmallows and Biscoff spread into the melted butter. Stir slowly until everything becomes smooth and creamy. The mixture should look thick, shiny, and fully melted without lumps.

Pour vanilla extract and salt into the melted marshmallow mixture. Stir again until combined. The vanilla adds warmth, while the salt balances the sweetness from the marshmallows and cookie butter.

Remove the pot from heat before adding the cereal. Pour Rice Krispies into the warm mixture slowly. Fold everything gently with a spatula until all cereal pieces are fully coated.

Sprinkle crushed Biscoff cookies into the mixture. Stir gently so the cookie crumbs spread evenly throughout the bars. This step gives extra crunch and deeper cookie butter flavor.

Transfer the mixture into the prepared baking dish carefully. Use a buttered spatula or clean hands to press the mixture gently into an even layer without packing too tightly.

Warm extra Biscoff spread for a few seconds until pourable. Drizzle it over the top of the treats using a spoon. This simple topping makes the bars look bakery-style and extra delicious.

Allow the treats to cool at room temperature for about 30 minutes. This helps them firm up properly before cutting. Do not refrigerate immediately because cold temperatures can make them hard.

Lift the treats from the pan using the parchment paper edges. Cut into squares or rectangles with a sharp knife. Serve fresh for the best chewy and crispy texture.