This fondant-covered cake decorated with hydrangeas is made of three graduated square tiers. The construction is the same as described for the buttercream cake. Before you begin working with the fondant, prepare all of the cakes by baking, filling, and crumb-coating each with a thin layer of buttercream icing. The tiers should be stored in the refrigerator at this point. This square cake calls for a square cake board; if you can't find one, have one cut out of flake board at a hardware store. Fondant must be at room temperature before you can use it. Once you've covered the cake with the fondant, do not refrigerate it; the fondant can become dewy. Finished tiers or the completed cake can stand at cool room temperature overnight.
1. Start by kneading the fondant as you would bread dough. Your work surface should be perfectly smooth; dust the area very lightly with cornstarch to keep the fondant from sticking. Knead the fondant until it feels soft and pliable.
2. On the same work surface, roll out the fondant; the ideal thickness is between 1/4 and 1/8 of an inch. If you notice any air bubbles as you roll the fondant, prick them with a clean straight pin. Roll out enough fondant to cover the entire top and sides of the tier you are working on; excess fondant can be reused for the remaining tiers as long as it has not come into contact with buttercream.
3. Lift the fondant with both hands, and lower it onto the prepared cake tier, working quickly so the fondant doesn't dry out.
4. Dust your hands with cornstarch if necessary, and smooth the top and sides of the cake, pushing out any air bubbles. The fondant will adhere to the buttercream crumb coat.
5. Run a pizza wheel or a knife along the edge of the cake, cutting away excess fondant. Complete process on remaining tiers, then assemble cake as described in Wedding Cake 101: How to Make a Buttercream Cake.