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Some icings, like a thin ganache, may take less time to set, but other icings, like a thick buttercream, may take longer. After 20 minutes, gently tap the outside of the cake with a clean hand to see if the frosting has hardened. If the frosting is still tacky, let the cake chill for another 5 minutes before taking it out of the refrigerator. It is easiest to apply most coatings with an offset spatula. Continue to coat the cake evenly until you reach a desired thickness of icing. Freshly applied icing will create a sticky surface for the sprinkles to adhere to.  If you are planning to roll your cake in the sprinkles, do not frost the top of the cake as you will have to hold the bottom and top of the cake to roll it.  If you want to use fondant, cover the cake with fondant as you normally would. Draw inspiration from some of the ingredients in your cake. If your cake has candied cherries in it, accent the outside with a white icing and vibrant red sprinkles. Also, note the color of your icing. Sprinkles that are a dark color may not be as noticeable against a dark chocolate icing as they would against a vanilla icing.  Pick sprinkles colors that complement seasonal and holiday cakes. Choose a mixture of orange, purple, and black sprinkles to decorate a pumpkin spice cake during the fall or around Halloween. Think about using red, pink, and white sprinkles for anniversaries or Valentine’s Day. Use nonpareils for a speckled rainbow look, or ovular jimmies for bigger splotches of color. Mix both sprinkle styles together to vary the visual texture. Use large confetti sprinkles or sugar coated chocolate sprinkles for a minimalist look or mix them with smaller sprinkles to make them stand out.

Summary:
Allow the crumb-coat to chill in the refrigerator for 20 minutes to harden. Cover your cake with a second, thicker layer of icing. Choose sprinkles that will complement the icing and the cake flavor.