Preheat the oven to 180°C (160°C fan forced).
Mix the cake mix, eggs, water and oil in a large bowl with electric mixer on LOW speed for 30 seconds, then on MEDIUM speed for 2 minutes (do not eat batter as it contains raw eggs). Pour the cake batter into well-greased 33cmx23cm rectangular pan.
Bake in the centre of the oven for 25-30 minutes or until skewer inserted into the centre of the cake comes out clean. Leave in the pan for 10 minutes. Run knife around sides of pan to loosen cake; remove from pan to cooling rack. Cool completely, about 1 hour. For easier handling, refrigerate or freeze cake 30-60 minutes or until firm.
Place the two tubs of frosting and the sachet of frosting from cake mix box into a large bowl and stir in green food colour until desired shade of green. Cut rounded top off cake with serrated knife to level surface; place cut side down. From the middle of one short side, make a diagonal cut to each corner on to opposite side, making 3 triangular pieces, as shown in diagram. Make tree shape by arranging 2 outer pieces with straight 33cm sides together on foil-covered tray. Frost top with some of the green frosting. Place remaining center piece of cake on top of frosted cake. Spread a thin layer of frosting over top and sides of cake to seal in crumbs. Refrigerate cake 30 minutes to harden frosting.
Frost cake with remaining frosting on top and sides. Pull a fork through the frosting on top of cake and sides to create “branches” all over. Using black writing icing tube pipe strings for the lights across top of tree. Place assorted colours of chocolate buttons on black string for lights.
Press candy cane pieces in end of tree to make trunk. Roll out yellow ready to roll icing and using a small star cookie cutter, cut out a star. Place star on top of the tree. Store loosely covered at room temperature.