Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line the base of a 1.5 litre ovenproof Pyrex glass bowl.
Make up 1 box of cake mix using the oil, water and eggs called for on the pack instructions. Pour the cake mixture into the bowl and level the top.
Bake in the centre of the oven for 1 hour 5 minutes-1 hour 10 minutes or until a round bladed knife inserted into the centre of the cake comes out clean. Remove from the oven and leave to cool on a wire rack for 10 minutes. Turn the bowl over onto the wire rack and leave the cake to fall out of the bowl naturally. Remove the bowl and leave the cake to cool fully. Peel off the baking paper.
Grease and line the cooled bowl again. Make up the second box of cake mix with the remaining vegetable oil, water and eggs according to the pack instructions, and bake as before in the bowl using the instructions above.
Carefully trim the top of each cake, to create a level surface for them to stick together. Also trim 1 cake on the smaller end so it has a larger base to sit on the cake board. Place this cake onto the cake board with the trimmed smaller end cut-side down.
In a small bowl, colour 100g of the icing with enough green food colouring to give a bright green colour and set aside. In a medium bowl, colour the remaining icing with the orange food colouring until it is bright orange.
Spread a scant quarter of the orange icing onto the top of the first cake with a palette knife. Top with the other cake, creating a pumpkin shape. Spread a thin layer of orange icing over the entire cake to seal in the crumbs. Chill for 45 minutes, then continue to ice the cake with the remaining orange icing. Using a palette knife, draw the knife up from the base of the pumpkin, going all around the cake. This creates the ridges of the pumpkin.
Frost the ice cream cone with the some of the green icing. Place the ice cream cone upside down onto the top of the cake for the stem. Place the remaining green icing into a disposable piping bag fitted with a writing piping nozzle. Pipe leaves and tendrils onto the pumpkin around the stem.
Dust a work surface with icing sugar and roll out the black icing to a thickness of 2-3mm. Cut out eyes, a nose and mouth, then stick these onto the cake to form a face.