Most truck drivers get into the business for the money or travel, but few walk away from it without an intense love for the machines themselves; rarely will you find a career driver who doesn't own a toy lorry, a Caterpillar diesel calendar or any number of branded T-shirts or apparel. Truckers take their profession to heart, which makes a tractor-trailer cake a natural thing for any driver's birthday party, a husband/wife team's anniversary or a new driver's Class A congratulations party.
Make cake batter according to package directions, grease the pan and bake the cake according to package directions. Allow the cake to cool, remove it from the pan and slice a thin layer off of the top to create a flat surface. You may need to make several pans to provide the building material for your cake.
Create your truck and trailer "chassis" out of corrugated cardboard. Start by cutting a rectangle to the length that you desire for your truck. An average truck is about 6 m (20 feet) long, 2.4 m (8 feet) wide, 4 m (13.5 feet) tall, has 90 cm (3 foot) diameter wheels with 60 cm (2 foot) diameter rims and about 30 cm (1 foot) of ground clearance. Scale these measurements down to whatever sized cake you require; a large 1/12-scale (1 inch equals 1 foot) cake would be enough for a 20-to-30-person party.
Repeat Step 2 to create the trailer chassis, but use a triple-thick layer of cardboard for the floor. The average trailer measures about 16 m (53 feet) long, 2.4 m (8 feet) wide, 4 m (13.5 feet) tall with about 90 cm (3 feet) of ground clearance and the same wheels as the truck. After you get your truck and trailer platform chassis cut out, create a scale 30 cm by 90 cm by 1.5 m (1 foot by 3 foot by five foot) "box" out of cardboard to support your tractor chassis. Build a scale 1.5 m by 1.5 m by 90 cm (5 foot by 5 foot by 3 foot) box to support your trailer chassis. Glue the trailer support to the bottom of the trailer chassis about 2/3 of the way back.
Place your icing container into a double-boiler to melt the icing to a near-liquid state. Keep the icing in this state ready for use.
Cut "building blocks" out of your cake and start staking them on the truck and trailer chassis to create the shape of the truck and trailer. pour icing over each of your blocks as a mortar to hold it together. Get as close as possible to the final shape, but bear in mind that the icing and fondant will add about 1 cm (3/8 inch) to the outside dimensions of your cake. Cut 18 scale 90 cm (3 foot) diameter by 60 cm (2 foot) thick rounds for your wheels. Use a paring knife to cut a scale 60 cm (2 foot) diameter and 30 cm (1 foot) deep hole out of the centre of 10 of them.
Pour icing over the entire truck and trailer, using a large knife or spatula to shape it into place or stick it to the sides of the trailer. Rough-shape the icing as it flows over the cake and cools. Repeat with the wheels. Set the truck, trailer and wheels into the refrigerator to cool for an hour.
Roll out your fondant sheets and use a paring knife to cut the appropriate shapes from it. Fondant is a kind of icing that contains gelatin so it will hold it's modelling-clay-like texture. Store-bought fondant can be expensive; check the References section below for a DIY fondant recipe using marshmallows and sugar. The DIY approach will allow you to tint the fondant to whatever colour you desire. working with fondant is just like working with modelling clay; if you're unfamiliar with it, then you might want to enlist the help of someone who is. Cover the truck, trailer and wheels with fondant, and sculpt it to whatever shape and detail level you desire.
Colour the truck with spray food colouring. Spray food colouring works just like spray paint, comes in many colours and metallics (including aluminium-look for the trailer) and offers about the same level of detail as spray paint. For a completely professional-level finish, you'll need to use a speciality airbrushing set-up with food colouring instead of paint.
Assemble the tractor, trailer and wheels on your serving platter. A 1/12 scale cake can get quite heavy, so consider serving it on a piece of plywood covered with waxed paper.
Some additional touches include real "glass" windows made out of melted sugar, LED running lights on the tractor and trailer, truck-trailer air lines made out of liquorice, rivets made from nonpareil ice cream toppings and even a number plate.