Using an electric hand mixer, beat together the butter and icing sugar until creamy. Fold in the flour and almonds, then beat in the egg yolk. Use a palette knife to bring the dough together, then shape into a ball with your hands. Wrap the dough in clingfilm and chill in the fridge for 20-30 minutes.
Preheat the oven to 180°C/fan 160°C/gas mark 4. Line a baking tray with baking parchment. On a work surface dusted with icing sugar, roll out the dough to a thickness of 5mm. Stamp out 24 flower-shaped biscuits using a flower-shaped cutter. Place on the baking tray and bake for 5-10 minutes. Leave to cool.
Beat together the butter and icing sugar. Divide between 2 small bowls and drop a small amount of pink food colouring into one bowl and green colouring into the other. Stir 1½ tsp water into each bowl. Sandwich half the biscuits together with pink icing, and half with green icing. Dust both sides with icing sugar before serving.