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In a mixer fitted with a paddle attachment, beat together butter, salt, and vanilla until light and fluffy.
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At low speed, gradually add the powdered sugar, one cup at a time. Add the heavy cream. Whip until smooth. Set aside 1 cup of buttercream in a small bowl and tint with a few drops of green food coloring.
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In another bowl, set aside another cup of buttercream and tint with a few drops of red food coloring. Leave the remaining buttercream as is.
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To Assemble Cake
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Place the first layer of confetti cake on a cake stand or serving platter.
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Put 1 cup of the green buttercream on top and spread evenly. Position the second layer of confetti cake and press to adhere.
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Put 1 cup of the red buttercream on top and spread evenly. Place the last layer of confetti cake and press lightly.
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Cover the entire cake with a thin coat of buttercream. This is what you call a “crumb coat”, it will ensure that there is no stray crumbs once you apply the final frosting on the outside.
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Refrigerate for 30 minutes to set.