In a saucepan, melt butter over medium heat. Add the mini marshmallows and mix until smooth. Once smooth, add vanilla and mix to combine.
Reduce the heat and carefully fold in the Fruity Pebbles, mixing until combined. Transfer half of the mixture to a parchment-lined casserole dish.
With wet hands or a silicone spatula, press down to compact the treats. Add a layer of the ice cream mixture, as thick as desired. Add crumble topping on top, if desired. Top with the remaining Fruity Pebble treat mixture, pressing carefully to smooth. Cover with parchment paper and freeze for at least 6 hours.
Freeze the remaining ice cream in a carton to enjoy on its own.
Once the ice cream sandwich is frozen, remove from the freezer and cut into evenly sized pieces. Wrap individually in parchment paper and store in the freezer. Enjoy!