Break the Chocolate into a small heatproof bowl and place over a saucepan of barely simmering water to melt. Remove from the water and leave to cool for 10 minutes.
Put the cake crumbs in a mixing bowl and mix in the melted Chocolate and jam until well mixed. Cover and chill for about 30 minutes until firm enough to shape.
Meanwhile, prepare the decoration. Lightly dust the work surface with icing sugar. Cut off a 15g (½ oz) piece of Marzipan and wrap in cling film. Cut off a further 40g (1 ½ oz) and wrap in cling film.
Take the remaining portion of Marzipan, flatten slightly and add 5 drops of Neon Orange Gel Food Colour on top of the icing. Fold the Marzipan over the Gel Food Colour to enclose it and carefully knead it into the Marzipan until evenly mixed. Continue adding Gel Food Colour, 5 drops at a time, until you achieve the desired colour, using icing sugar to dust your fingers if the Marzipan becomes sticky. Wrap in cling film to prevent it from drying out. Repeat the process to colour the 40g (1 ½ oz) piece with Lime Green Gel Food Colour, and the smallest piece with Jet Black Gel Food Colour. As a guide, you will require a whole tube of Orange, about 30 drops Green and 45 drops Black to achieve the most vibrant shades.
Divide the truffle mixture into 8 equal portions. Form into balls and place on a plate lined with baking parchment. Cover and chill for a further 30 minutes until firm.
Divide the orange Marzipan into 8 pieces. Working on one portion at a time, flatten into a circle about 9cm (3 ½ inch) diameter and lay a chocolate truffle ball in the centre. Draw up the edges of the circle to enclose the truffle, pinching the top together to completely cover. Roll the Marzipan ball carefully in your hands to smooth it into a flattened ball shape. Carefully score the sides with a small knife and place on a board lined with baking parchment whilst you cover the remaining chocolate truffle mixture.
When you have made 8 pumpkin shapes, roll out the green marzipan thinly and use to make leaves, stalks and curly vine-like stems for each pumpkin. Secure on top using a little water if necessary.
Finally, break off tiny pieces of black marzipan and form eyes and mouths, and gently press on to the pumpkins to make faces. Once you’ve assembled all the pumpkin truffles, they are ready to serve and enjoy!
Note: If the room temperature is warm, put the truffles in the fridge for 30 minutes before serving to firm up.