Light and fluffy, this frosting is a sugar-free version of the classic Swiss buttercream. It’s both easy to spread as well as sturdy enough to pipe, so don’t hold back on decorating that extra-special low-carb, keto, or sugar-free celebration cake or cupcakes.
In a large, clean mixing bowl (or the bowl of your stand mixer), combine the egg whites, Baking Sugar Alternative, and salt.
Place over a saucepan of simmering water (you should just barely see some lazy bubbles coming up) and whisk constantly until the sugar alternative has dissolved and the mixture is frothy and measures 161°F (71.7°C) on a digital thermometer.
Transfer the bowl to your mixer fitted with the whisk attachment. Whip the meringue mixture at medium-high speed until it’s thick and glossy and the bottom of the bowl is no longer warm, 5 to 6 minutes. At this point the meringue should have cooled to about 90°F.
With the mixer running add the butter a tablespoon at a time, mixing each in fully before adding the next.
Once half the butter is in the bowl, stop and scrape the sides and bottom to make sure all the meringue is incorporated. Then finish adding the rest of the butter, beating until the frosting is smooth and creamy.
Beat in the vanilla.
Spread or pipe the frosting as desired. For best results, wait until just before serving before frosting your cake or cupcakes; the buttercream does have a tendency to break (or separate) over time, especially in humid weather.
Store the frosting for up to 1 week in the refrigerator, or up to 6 months in the freezer. Let it come to room temperature before using, whipping it gently if necessary to bring it back together.
Make sure your mixing bowl is very clean and totally free from even a speck of grease, which can affect the expansion of egg whites during whipping.
This recipe was developed using King Arthur Baking Sugar Alternative. Using another sugar replacement or alternative, one with a different formulation, will yield different results. For best success, use the specific product called for: King Arthur Baking Sugar Alternative.
To make chocolate buttercream: Melt between 1/2 cup and 2/3 cup (85g to 102g) unsweetened baking chocolate; set aside to cool to lukewarm. Prepare the frosting through the vanilla being added (step 6); for added chocolate flavor, dissolve 1/4 teaspoon espresso powder in the vanilla before adding it to the buttercream. Beat in the melted chocolate; if your buttercream is a bit cool and looks compact instead of fluffy, use slightly warmer chocolate to achieve a glossy, spreadable frosting.
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