Melt the butter along with the white chocolate in a pan set over simmering water. Stir continuously until it becomes smooth, then let it cool for about 10 minutes.
Next, incorporate the crushed biscuits, mini marshmallows, and mixed candies. Press this mixture firmly into a lined small roasting pan.
Melt the remaining white chocolate, then drizzle it over the pressed mixture. Add your festive decorations on top!
Chill everything in the fridge for roughly 2 hours. Once set, take it out and slice into delightful chunks.