To begin, line the base and sides of a 20 cm cake tin with baking paper - making sure you leave the paper hanging over the edges.
In a large bowl, add the rice bubbles and set aside until needed.
Place the butter and marshmallows in a small saucepan and cook over a medium heat stirring occasionally until they have both melted and combined.
Pour the melted mixture quickly into the bowl with the rice bubbles, and mix until combined.
Pour the combined mixture into a slice tin and use a metal spoon to spread across the tin and push down.
Sprinkle 100s and 1000s over the top (optional) and place in the fridge for two hours or until set.
Once set, remove the slice from the fridge and cut into small squares.