Melt the milk chocolate following the instructions on the back of the packet. Put one tablespoon of chocolate into each baking case and keep the remaining chocolate for later. Spread some chocolate from the bottom, towards the top covering the entire inside surface of the baking case. This needs to be even so check there are no holes or thin spots. Once they are all coated, place in the fridge until solid. To speed this up you can put them into the freezer for a few minutes. 

Break the extra dark chocolate into a heatproof bowl. Add the water and place into the microwave on a high heat for 1 minute. Allow to stand for two minutes then stir until the chocolate is completely melted. Alternatively melt the chocolate over a saucepan of simmering water. 

Add in dried skimmed milk powder and arrowroot to the chocolate mixture and stir well. 

Fill each chocolate baking case with the dark chocolate mixture. Place into the freezer for five minutes. 

Re-melt the remaining milk chocolate and spread a thin layer on top of the dark chocolate filling. Then return to the freezer for a few minutes until set. 

Put the heart marshmallows into a heat proof bowl and microwave on a 50% power for 1 minute. Stir until melted then spoon into a piping bag fitted with a star nozzle. Pipe a swirl of the marshmallow topping on each cupcake and sprinkle with chocolatey caramel crunch. 

To serve, simply peel off the baking case wrapper and place the hot chocolate bomb in a large mug. Into a saucepan heat 200mls of whole milk until just about boiled. Then pour over the chocolate cupcake bomb. Leave to stand for three minutes and then stir until completely melted.