Adjust oven rack to the middle position. Preheat oven to 180°C. Butter a 30 cm x 21 cm swissroll pan generously and line base with baking paper. Beat eggs, oil and melted butter together for 30 seconds on high speed until well blended. Add hot milk and beat briefly on high speed. Add the contents of the pack and gently mix in by hand with a spatula. Mix only until combined. It is important not to over mix. Pour the mixture into the lined swissroll pan and level. Place in oven and bake for 20 -25 minutes. Cake is done when it pulls away slightly from the sides of the pan. Cool cake in the pan for 10 minutes. Turn out onto a wooden board. Copy the pattern attached and mark it off on the underside of the cake.
Mix icing according to package instructions. Beat in green colour.
Cut the cake according to pattern. Cut into three lengthwise strips each measuring 7 cm wide, then cut each strip into three triangles, each 10 cm wide. Top and tail the triangles so that there is only a half triangle of cake left at the ends of each strip, then glue half triangles together with a little icing. Pipe the tops of the cakes with horizontal icing lines. Arrange the cake trees on a board, then glue chocolate flake with icing at the base of each tree for the trunk. Top each tree with a chocolate star. Add smarties and sprinkles for decorations.