Italian Meringue Buttercream

Before you start

In Italian meringue buttercream, the whites are whipped while a sugar syrup boils separately, and the syrup is poured in at the soft-ball stage, about 118 to 121°C (245 to 250°F). The hot syrup cooks and stabilizes the foam, giving a dense, very fine meringue that accepts a large amount of butter. The result is the most stable of the buttercreams: smooth, only lightly sweet, and less likely to soften on a warm day or under fondant.

Timing is the skill. You want the whites at soft peaks just as the syrup reaches temperature, so start whipping when the syrup is around 110°C (230°F). Pour the syrup down the side of the bowl in a thin stream with the mixer running, avoiding the whisk, or it spins into hard threads. After that, the method follows Swiss buttercream: cool the meringue fully, then beat in cool, soft butter until it comes together.

Ingredients

• Syrup200 g (1 cup) granulated sugar
• 60 ml (¼ cup) water
• Meringue150 g (about 5 large) egg whites — room temperature
• ¼ tsp cream of tartar
• 50 g (¼ cup) granulated sugar
• Finish400 g (1¾ cups) unsalted butter — soft but cool, about 18°C (65°F), in tablespoon pieces
• 2 tsp vanilla extract
• ¼ tsp fine salt

Method

1. Put the egg whites and cream of tartar in the clean mixer bowl. Have the remaining 50 g sugar measured beside the mixer.
2. Stir the 200 g sugar and the water in the saucepan over medium heat just until the sugar dissolves. Stop stirring, clip on the thermometer and brush down any crystals on the pan walls with a wet pastry brush.
3. When the syrup reaches about 110°C (230°F), start whipping the whites on medium speed. Once they are foamy and holding soft peaks, add the 50 g sugar gradually and keep whipping.
4. When the syrup reaches 118 to 121°C (245 to 250°F), take it off the heat. With the mixer on medium, pour it in a thin, steady stream down the inside wall of the bowl, avoiding the whisk.
5. Increase to medium-high and whip until the meringue is thick and glossy and the bowl has cooled to body temperature or slightly below, about 15 minutes.
6. Switch to the paddle. Add the butter a piece at a time on medium speed. The mixture may thin out or look broken partway through; continue until it becomes thick and smooth.
7. Beat in the vanilla and salt for another 2 minutes, then run the mixer on low for a minute or two to smooth out air pockets before frosting.

Tips for success

• If the whites reach soft peaks before the syrup is ready, turn the mixer to its lowest speed to keep them moving without overwhipping.
• If syrup splashes and hardens on the bowl, leave it; scraping those threads in makes lumps.
• This frosting is the best choice under fondant or for cakes that will sit outdoors for a short while in mild weather.

Substitutions

Cream of tartar

Use ½ tsp lemon juice to stabilize the whites, or leave it out; the meringue will be slightly less sturdy but the hot syrup still sets it.

Fresh egg whites

Pasteurized carton whites work if you want extra assurance, though they can take longer to reach soft peaks.

Vanilla extract

A few drops of orange blossom water, a spoon of liqueur, or vanilla bean paste for visible specks.

Variations

Salted caramel Italian buttercream

Beat in 120 ml (½ cup) cooled, thick salted caramel sauce at the end, a spoonful at a time.

Raspberry Italian buttercream

Beat in 80 g seedless raspberry purée reduced by half and cooled.

Troubleshooting

Syrup crystallized in the pan

It was stirred after boiling or sugar crystals on the pan walls fell back in. Start again, stop stirring once it boils and brush the sides with water.

Hard lumps of sugar in the meringue

The syrup hit the whisk and spun into threads. Pour against the side of the bowl next time; sieve out lumps you cannot beat smooth.

Buttercream is runny

The meringue was still warm when the butter went in. Chill the bowl 15 to 20 minutes and beat again.

Buttercream is curdled

It is too cold. Warm the outside of the bowl briefly while beating until it turns smooth.

Storage

Refrigerate covered for up to 1 week or freeze for up to 3 months. Bring to room temperature and re-beat before using. Cakes frosted with it hold well at cool room temperature for a day.

Safety notes

The hot syrup heats the whites considerably, but this method does not measure a set egg temperature. For anyone at higher risk from foodborne illness, use pasteurized egg whites. Contains egg and milk.