Line a deep baking dish (about 30x20cm) with baking paper. Add marshmallows and butter to a very large pot and melt on medium heat.
Once melted, remove from heat, add rice bubbles and stir well with a spatula until combined. Pour into prepared dish, flatten with the back of the spatula and place in the fridge to set for about 30 minutes.
In a small pot, add about 3cm of water. Bring to a boil. Break or chop up chocolate and add to a heat-proof bowl along with oil. Place bowl on top of boiling pot, making sure no water touches the bowl and reduce heat to medium. Stir continuously until chocolate has melted. If your chocolate has split, add a little more oil and stir well until smooth again.
Separate chocolate into 3 bowls and add a couple drops of food colour to each bowl. Stir well.
Remove rice bubble cake from the dish and cut into rectangular shapes (it's OK if they're all different!). Use a spoon to cover the top half of each rectangle in chocolate, making a few of each colour.
Lay out on a piece of baking paper to dry. While drying, place mini marshmallows on for eyes and chocolate drops for the pupils.