Rectangular cakes, also referred to as sheet or slab cakes, have a variety of decoration potential due to their shape and size. Their large surface area gives you more space to write messages, draw pictures or create elaborate frosting designs than square or round cakes. Learning how to decorate a rectangular cake takes practice, so be sure that you give yourself one or two trial runs before you present the cake to party goers of a bridal shower or birthday party.
Remove the cake from the oven and allow it to cool. Make sure the cake is cool before you begin decorating it, or else the cake can collapse from the heat. If you do not have the time to wait for it to cool on its own, transfer the cake from the oven to the fridge or freezer to bring the temperature down quickly.
Prepare a clean platter or cake stand and coat the bottom with parchment paper. Transfer the cake from the baking dish to the platter.
Use a knife to trim the untidy edges from around the cake so that everything looks smooth. Attempting to frost a cake with an uneven surface will cause the icing or frosting to be uneven, too.
Trim a featherweight bag to fit a 789 tip. This is a large tip that will help you apply large sections of icing at a time. Fill the featherweight bag with icing.
Apply the icing onto the cake using the featherweight bag. Squeeze the featherweight bag while holding it at a 45 degree angle. Select a coating direction, such as vertically or horizontally. Keep this direction consistent while you frost the cake. Do not worry about skipping over areas or making uneven lines because you will go back to fill those in.
Use your offset spatula to smooth over the icing. Keep the direction of your smoothing motion consistent to avoid noticeable lines.
Fill a disposable pastry bag with more icing, perhaps in a different colour, which you will use for creating swirl piping around the edges of your sheet cake. Fit the 1M Open Star Decorating Tip onto the decorating bag and begin piping. To make one continuous line, apply pressure as you move your arm down the edge of the cake. If you wish to make individual piping swirls, apply pressure for a second and then release pressure. Continue the apply and release technique until you have covered all the areas that you want to have the icing swirls.
Use a different colour icing to write a message in the centre of the rectangular cake. Fill a second pastry bag with the new icing and squeeze the bag as you write out each letter. Do this part slowly so that the letters do not run into one another from the icing.