To make the dough, tip the butter, flour and a good pinch of salt into a food processor. Blend until the mixture resembles breadcrumbs, and the butter is well mixed in. Add the icing sugar, egg yolk and 2 tsp cold water. Blend again until the mixture starts to clump together, add another 1 tsp water if the dough looks too dry, but try to avoid adding any more, as this will make the biscuits tough. Tip the crumbs out onto a work surface and squash everything together to make a ball of dough, you may have to knead it a few times for an even texture. Wrap in cling film, pat into a flat disk and chill for 30 mins.
Heat oven to 180C/160C fan/gas 4 and line 2 large baking sheets with baking parchment. If the dough is very firm, leave it at room temperature for 10 mins, or until softened a little. Dust your work surface with flour, then unwrap and roll out the dough to the thickness of a £1 coin. Stamp out as many skull shapes as you can, then squash the scraps of dough back together, reroll and stamp out a few more. Arrange the skull shapes over the trays, bake for 18-20 mins, until just golden, swapping the trays over halfway through if your oven cooks unevenly. Leave to cool on the trays for 10 mins, then transfer to wire racks to cool completely.
Whilst the biscuits cool, prepare the icing. In a large bowl, mix the royal icing sugar with enough water to make a thick icing. Divide the icing into as many bowls as the number of colours you’d like to use (you’ll need to colour 1 batch black for the mouth) then use the food colouring to colour each a vivid shade - a tiny drop of the food colouring paste goes a long way, so start with a little, then add more if you like. If the icing is too thick to pipe, add a drop more water. Transfer each colour to a separate disposable piping bag and secure the ends (clothes pegs do a good job of keeping the ends closed.)
Break off 50g of the sugarpaste and set the rest aside, wrapped in clingfilm (This will prevent it from drying out and cracking.) Use the red food colouring to dye the small lump of sugarpaste red, kneading until evenly coloured. To make the roses, take hazelnut-sized balls of the red sugarpaste, and press them into long slim oval shapes on the work surface (roughly 1cm x 4 cm, use a little icing sugar on the surface if it gets too sticky.) Roll up the sugarpaste from one end to create a rose. Whilst still on its side, cut off the bottom to give you a flat base. Continue with the rest of the red sugarpaste until you have about 20 roses (enough for 4 biscuits.)
Clean down the surface, and dust with a little extra icing sugar. Roll out the remaining sugarpaste to the thickness of a 50p piece, then cut out as many skull shapes as you have biscuits. Use a little of one of the coloured icings to stick the sugarpaste skulls to the biscuits.
Scrunch any sugarpaste scraps back together and dye the ball of sugarpaste black with the black food colouring, kneading until you have an even colour. Roll out the black sugarpaste and stamp out circles for the eyes (use a round cutter, or the end of a piping nozzle.) Stick the eyes and roses to the biscuits with a little of the coloured royal icing. Snip off the corner of all the piping bags to make a tiny opening and decorate the biscuits as you wish – flowers, hearts and dotty designs all look good. Leave the biscuits to dry for 1 hr before serving. The decorated biscuits will keep for 3 days in a sealed container.