Use immediately per the decoration instructions in the blog post. Buttercream can be stored in the refrigerator in an airtight container for up to 5 days; if storing for longer, transfer to a ziptop bag and freeze for up to 3 months. To reuse, scrape the chilled buttercream in the top pan of a double boiler (or, make a homemade version by placing a heatproof bowl over a sauce pan filled with 2 to 3 inches of simmering water, ensuring that the bottom of the bowl does not touch the water) and cook over medium-low heat until the frosting has melted all around the edges, with most of it still in a cold and solid mass in the center. Transfer to a stand mixer fitted with the whisk attachment, and whisk on low speed to prevent splashing for 1 minute. Gradually increase to high and whip until homogeneous, about 4 minutes.