Line a 9-inch x 9-inch square pan with parchment paper.
In a large pot or saucepan over medium heat, melt the butter.
Once melted, add the vanilla extract and stir.
Add the marshmallows and continue stirring until they melt into the butter completely.
Pour in the cereal and crushed Biscoff cookies, stirring until well combined.
Remove from heat, transfer the mixture to the prepared pan, and use a rubber spatula to spread it evenly. Avoid packing it too tightly to keep the treats fluffy. If it sticks, dab a bit of water on the spatula or your fingers.
Melt the chocolate according to package instructions, either in the microwave or using a double boiler. Spread it evenly over the Rice Krispies treats in a thin layer.
Drizzle the Biscoff spread in lines across the chocolate layer. Use a toothpick to create a swirled design by dragging it through in alternating opposite directions.
Sprinkle more crushed Biscoff cookies on top if desired.
Chill in the fridge for 15 to 20 minutes until the chocolate sets.
Cut into 16 squares and press the bottom into more Biscoff crumbs for extra flavor. Then, indulge in the deliciousness!