Add marshmallows to the butter and stir until they are completely melted. I have bought the home brand pink and white ones and they work fine, but I think the taste is nicer if it is just plain white.
Pour marshmallow mix into the bowl with rice bubbles and combine well.
Press mixture into the tray. (If it is sticking to your hands, try wetting them slightly with cold water.) You can sprinkle with 100s and 1000s if desired
Put in the fridge to cool and set.
Once set, cut into desired shape. I tend to go for rectangles about 5cm x 3cm.
Recipe by Planning With Kids at https://planningwithkids.com/2009/08/30/rice-bubble-slice-recipe-and-family-menu-plan/