Separate half of the buttercream into a separate bowl, and add 2-3 drops of blue food colouring, stirring until smooth. You can then add an extra drop of blue and one of green and stir slightly to give a multitone effect.
Add the un-coloured half of the buttercream to a piping bag with a round nozzle. Pipe this in smooth lines across half of each the cupcake in a single layer.
Crumble the excess cupcake chunks from earlier into little crumbs, and press these into the buttercream halves of the cupcakes to look like sand.
Add the blue frosting to a piping bag with a flat piping tip. Hold this so the line is vertical and pipe the blue icing in zigzags across the plain half of the cupcakes to look like waves.
Take your seashell mould and brush icing sugar into the cavities then press your white fondant into the cavities before gently popping them out. Repeat until you have enough seashells for a couple per cupcake. Place these onto the sand side of your cupcake.
Melt your white chocolate in the microwave, 30secs at a time, stirring between bouts until smooth.
Pour you chocolate into the mermaid tail mould and set in the fridge for 10mins. Pop this out carefully, brush with lustre dust (optional) and top your cupcake with the tail. Repeat until you’ve topped as many cupcakes as you’d like.