Grease and flour a 13x9-inch (33x23-cm) cake pan.
Preheat oven to 350°F (175°C).
To make cake, prepare lemon cake mix with eggs, water and oil if required, according to package instructions.
Spread batter evenly in cake pan.
Bake 35 to 40 minutes, following package instructions to test doneness.
Cool as indicated on package.
Cut cooled cake horizontally into 2 layers; place 1 layer on top of the other.
Using a cardboard pattern, cut out the shape of a car from the cake, cutting through both cake layers.
Spread 1 layer with jam.
Cut out windows in the other layer; place on top of first layer.
Cut out 2 extra wheels from cake trimmings.
Place on top of car wheels using jam to join.
To make frosting, put powdered sugar into a small bowl.
Beat in enough water to give a coating consistency.
Cover car with about two-thirds of the white frosting.
Stir 1 to 2 teaspoons cocoa powder into remaining frosting.
Make a small pastry bag from baking parchment
Fill pastry bag with chocolate frosting; outline car as illustrated.
Decorate car with silver cake decors and M&M's while frosting is soft.