Line a baking tray with baking paper. I used a lamington sized tin.
In a saucepan on a medium heat, melt marshmallows, butter and vanilla together. This will take around 5 minutes.
Once marshmallows have melted, add fruit loops and melted marshmallows in a large bowl and mix until combined.
Add fruit loop mix to the baking tray and push down with a spatula ( you can wet the spatula to help with sticking). The mix will be VERY sticky, but this goes once the slice is set. I use the back of another tray to flatten down the slice.
Add sprikles to the top of the slice and allow to set in the fridge for 3-4 hours.
Slice and store in an airtight container in the fridge for 3-4 days.