Preheat the oven to 180ºC, fan 160ºC, gas 4. Grease and line a deep 18cm loose-bottomed round cake tin with baking parchment.
Mix the cocoa powder with 150ml boiling water and leave to cool slightly. Mix this and all the cake ingredients together in one go in a mixing bowl.
Grease a 750ml round Pyrex bowl with a little sunflower oil, and place a 3cm disc of greaseproof paper in the bottom. Pour in 275g of the cake mixture and cook in the microwave at 360W for 6 minutes. Immediately remove from the microwave and turn out onto a wire rack to cool completely. Repeat using another 275g of the mixture.
Meanwhile, pour the remaining cake mix into the prepared tin and bake in the oven for 1 hour 20 minutes or until a skewer comes out clean. Leave to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
To make the buttercream, beat the butter in a bowl until soft. Gradually add the icing sugar until combined, add the vanilla extract and beat until smooth. Take one third of the icing and transfer it to another bowl. Beat the red and yellow food colouring into the smaller portion to obtain a vibrant orange colour, then beat the cocoa powder and milk into the remaining two-thirds.
Once they are cool, set one of the two microwave cake halves, rounded side down, onto an upturned plate. Sandwich the other half on top with a little of the orange buttercream. Using a palette knife, cover the ball of cake with orange icing, and then coat again to hide any cake crumbs. Finish with vertical sweeping motions to mimic pumpkin skin.
Using a long serrated knife, level the top of the large circular cake and place it on a platter or flat cake plate. Cover the cake with one layer of chocolate buttercream, then sparingly sprinkle and pat the crushed bourbon biscuits over the top and sides of the cake (leave enough exposed buttercream to stick on the leaves and vines).
Push the wooden skewer vertically into the centre of the cake. Lift and carefully lower the pumpkin onto the skewer using two forks pushed into the bottom third. Smooth over any smudged icing with a palette knife. Decorate with the cola laces, snipping any excess lace once on the cake with a pair of scissors.
Roll out the green icing and use a sharp knife to cut out and fashion leaves. Roll into thin sausages for the vines and make a pumpkin stalk. Roll out the black icing and use the sharp knife to cut out the eyes, nose and mouth use our pumpkin template as a guide. Stick these to the pumpkin to give it a face, and the greenery round the base.