Line an 8x8 pan with parchment paper, leaving enough extra parchment hanging over the sides so you can easily lift the bars out later. Lightly spray the parchment with nonstick spray if desired.

In a medium saucepan over medium heat, melt the butter and mini marshmallows together. Stir frequently with a silicone spatula until the mixture is completely smooth and no marshmallow lumps remain. Be careful not to overheat the mixture or the treats can become hard instead of soft and chewy.

Add the melted Biscoff cookie butter spread to the marshmallow mixture and stir until fully combined. The mixture should look glossy, creamy, and light caramel in color with that warm spiced Biscoff flavour.

Remove the pan from the heat and immediately fold in the Rice Krispies cereal. Stir gently until every piece of cereal is evenly coated. Work fairly quickly here so the mixture does not start to set before pressing it into the pan.

Transfer the mixture to the prepared pan. Use a lightly greased spatula or piece of parchment paper to gently press the cereal mixture into an even layer. Do not firmly pack it down or the bars can turn dense and hard instead of light and gooey.

Melt the white chocolate in a microwave-safe bowl in 20 second intervals, stirring between each interval until smooth. Pour the melted white chocolate evenly over the top of the bars.

Warm the remaining Biscoff spread until it reaches a smooth pourable consistency. Drizzle it over the melted white chocolate, then use a knife or skewer to gently swirl the two together for a bakery-style finish.

Refrigerate the bars for about 30 minutes or until fully set. Lift the bars from the pan using the parchment paper handles, then slice into squares with a sharp knife. For the cleanest cuts, wipe the knife between slices.