Prepare the 8-inch square pan with grease and parchment across the bottom and sides, leaving an overhang, then microwave the white chocolate, Biscoff spread, and butter together in 10-second bursts, stirring thoroughly after every burst. Stop heating once stirring produces a glossy, ribbon-smooth mixture without solid chocolate pieces.

Fold the mini marshmallows, chopped Biscoff cookies, and fudge pieces through the melted coating with a spatula. Keep turning the mixture until the mix-ins look evenly coated and distributed, without crushing the cookie chunks.

Pack the mixture into the lined pan, pressing firmly into the corners and leveling the surface, then refrigerate for 1 hour. The slab should feel firm enough to lift with the parchment rather than sag or pull apart.

Lift out the slab, cut it into 16 bars, arrange them on a cooling rack over newspaper, then microwave the reserved 2 tablespoons of Biscoff spread in a microwave-safe piping or food bag for 20 seconds, snip a small opening, and drizzle zigzags over the bars. Let the decoration set completely so it stays in place when you handle the bars.