You don't need to wait until you are ready to ice your cake to make the buttercream frosting. Combine the butter, sugar and flavouring into an icing ahead of time and place it in the refrigerator. Before you are ready to use it, set the icing on the counter and allow it to reach room temperature. If it's still too stiff to use, simply warm the buttercream or add corn syrup to make it pliable.
Place the buttercream in a metal bowl.
Fill a saucepan with water and bring to a boil.
Place the metal bowl full of buttercream on top of the pan with the boiling water. The indirect heat will begin to melt the butter and sugar.
Stir the buttercream vigorously as it loosens, becoming spreadable. Remove the bowl of buttercream when it is smooth and before it melts into a liquid.
Place the buttercream in a bowl.
Drizzle light corn syrup in the bowl.
Stir the buttercream. Drizzle and stir more corn syrup into the mixture as necessary until the buttercream is no longer stiff.