Heat a large non-stick pan over a medium heat and add the rice pops. Toast them for 4-5 minutes, stirring occasionally, until lightly toasted. Remove from the heat and set aside.
Heat the butter in a large saucepan over a medium heat and cook until it begins to brown and gives off a nutty smell. Lower the heat and add the marshmallows. Cook for a further 2 minutes, stirring constantly, until the marshmallows have melted. Remove from the heat and stir through the toasted rice pops.
Line a large baking sheet with greaseproof paper and spread the mixture out to a large rectangle, roughly 40 x 28cm. Leave to cool for 20 minutes and allow them to firm up, then trim the uneven edges and cut into 48 rectangles, roughly 3½ x 5½cm.
It's time to decorate your scary monsters! To make the Frankenstein monsters, melt 100g white chocolate chips in a heatproof bowl over a pan of simmering water. Mix with the green food colouring until you have a bright green colour. Dip 12 of the bars in the green chocolate and leave on a tray lined with greaseproof paper in the fridge to set. Once set, create some hair in a zig-zag effect using the black writing icing pen. Mix the icing sugar with ½ tbsp water until a thick icing is formed then pipe 2 blobs of this on each rice pop and top with a small drop of black writing icing to create eyes. Finish with a zig-zag line to make a mouth.
For the vampires, melt 100g white chocolate in a bowl over a pan of simmering water and dip 12 of the bars in it to coat the surface. Place in the fridge to set, then draw a triangular hairline on the top and a line for the mouth. Use the icing sugar mix to pipe on eyes and pieces of strawberry laces to create fangs.
For the mummies, melt 25g white chocolate in a bowl over a pan of simmering water and place in a piping bag. Drizzle the chocolate over 12 of the bars to create a striped effect. Set aside in the fridge to cool, then create eyes with the icing sugar mix and black writing icing.
For the plain monsters, melt the remaining 75g white chocolate in a bowl over a pan of simmering water. Divide this chocolate into two bowls and colour one with yellow and one with blue colouring. Dip the tops of 6 bars into the yellow colouring and the tops of 6 into the blue colouring so that it coats roughly the top third of the bars. Leave these in the fridge to set, then create eyes with the icing sugar mix and black writing icing.