Preheat the oven to 190°C / 375°F / Gas Mark 5. Lightly spray 2 x 20cm Victoria sandwich tins with cake release spray.
Break the white chocolate into a heatproof bowl and place over a saucepan of barely simmering water until melted. Remove from the water and cool for 10 minutes.
Sift flour and baking powder into a mixing bowl, and stir in the ground pistachios and sugar. Make a well in the centre.
Gradually mix in the eggs, melted butter and milk until well blended then add the almond extract, melted white chocolate and gel food colour – approx. 70 drops to make a pretty shade. Mix well until thoroughly blended.
Divide the mixture between the cake tins, smooth the tops and bake in the oven for about 22 minutes, until risen, firm to the touch and lightly golden. Leave to cool for 5 minutes then turn on to a wire rack to cool completely.
To decorate, put the butter in a bowl and gradually sieve and mix the icing sugar into the butter to make a smooth, creamy icing.
Spread one of the cakes thickly with the butter icing and sandwich together with the other cake half. Place on a board or serving plate. Brush the top and sides of the cake with apricot jam.
Lightly dust the work surface with icing sugar and knead the marzipan until soft and pliable.
Roll out to form a round approx. 30cm (12inch) diameter. Use your rolling pin to help you lift the marzipan and drape over the cake.
Smooth into position the over the cake using the palm of your hand, working from the centre to the edges and down the sides. Trim away the excess marzipan.
Lightly dust the work surface with icing sugar and knead the regal-ice until soft and pliable. Cut 100g off the regal-ice and wrap in cling film to prevent it from drying out.
Divide the remaining 400g into 3 pieces and flatten each slightly.
Add 5 drops of green gel food colour on top of each piece of icing. Fold the icing over the gel to enclose it and carefully knead it into the icing until evenly mixed.
Add a further 5 drops green gel food colour to each portions and work in as above.
Combine all 3 pieces of icing together to make a pretty pastel green shade. Wrap in cling film to prevent it from drying out.
Lightly dust the work surface with icing sugar and roll out the green icing to form a round approx. 32cm (12 ¾ inch) diameter. Use your rolling pin to help you lift the rolled icing to drape over the cake. Smooth into position using the palm of your hand, working from the centre to the edge and down the sides. Trim away the excess icing.
To achieve an extra smooth finish, dust your hand with a little icing sugar and gently rub the cake all over to buff the surface.
From the 100g piece of icing, take off 15g (½ oz) and colour with the yellow gel food colour in the same way - you will require about 15 drops to achieve a vibrant shade.
Lightly dust the work surface with a little icing sugar and roll out the remaining white regal-ice thinly.
Cut out daisy shapes using assorted sized cutters, re-rolling as necessary. Brush the top of the cake lightly with a little water to secure the daisies in place.
Break off small pieces of the yellow regal-ice and form into balls. Secure in the centre of each daisy using a little water.
For a finishing touch, secure a length of ribbon round the base of the cake.
Serve and enjoy!