Swiss Meringue Buttercream

Before you start

Swiss meringue buttercream starts with egg whites and sugar stirred over simmering water until the sugar dissolves and the mixture is hot. Whipped into a thick, glossy meringue and then loaded with soft butter, it becomes a satiny frosting that is much less sweet than powdered-sugar buttercream. It spreads into razor-smooth sides, takes color evenly and tastes mostly of butter and vanilla.

Nearly every problem comes down to temperature. The meringue must cool to about room temperature before the butter goes in, or the butter melts and the bowl fills with soup. The butter should be soft but still cool, roughly 18°C (65°F). Somewhere midway the mixture usually looks curdled or broken; this is normal, and steady beating brings it together. If it does not, gentle warming or chilling, depending on the symptom, fixes it almost every time.

Ingredients

• 150 g (about 5 large) egg whites — no trace of yolk
• 200 g (1 cup) granulated sugar
• 340 g (1½ cups) unsalted butter — soft but cool, about 18°C (65°F), cut into tablespoon pieces
• 2 tsp vanilla extract
• ¼ tsp fine salt

Method

1. Wipe the mixer bowl and whisk with a paper towel moistened with vinegar. Any grease keeps the whites from whipping to full volume.
2. Bring 3 to 4 cm (about 1½ in) of water to a bare simmer in the saucepan. Whisk the egg whites and sugar in the mixer bowl and set it over the pan without letting the bowl touch the water.
3. Whisk gently and constantly until the mixture reaches 71°C (160°F) on the thermometer and feels completely smooth when rubbed between your fingers, 5 to 8 minutes. It will look thin and foamy.
4. Move the bowl to the mixer and whip with the whisk on medium-high until the meringue is thick, glossy and holds stiff peaks, and the outside of the bowl feels neutral rather than warm, 10 to 15 minutes.
5. Switch to the paddle. On medium speed, add the butter one piece at a time, waiting until each is mostly absorbed. Around the halfway point the mixture may thin out or look curdled; keep going.
6. Add the vanilla and salt and beat on medium-high for 3 to 5 minutes, until the buttercream turns smooth, thick and silky. Finish on the lowest speed for 2 minutes to push out large air bubbles.

Tips for success

• If you lack a thermometer, the mixture is usually ready when it is hot to the touch and no sugar grains remain, but checking the temperature is the reliable way to reach 71°C (160°F).
• Flavor at the end: melted and cooled chocolate, fruit purées reduced to a jam, or citrus curd beaten in a spoon at a time all blend well.
• Use the frosting at room temperature. Straight from the refrigerator it is hard and waxy and will tear a cake.

Substitutions

Fresh egg whites

Pasteurized carton whites work but often whip more slowly and to slightly less volume. Avoid whites labeled as containing additives that prevent whipping.

Granulated sugar

Superfine sugar dissolves faster over the heat. Brown sugar gives a caramel note and a beige color.

Unsalted butter

Salted butter works; leave out the added salt. Plant-based butter sticks can work but the frosting is softer and needs chilling.

Variations

Chocolate Swiss meringue buttercream

Beat in 170 g bittersweet chocolate, melted and cooled to about 30°C (86°F), at the end.

Brown sugar and cinnamon

Use light brown sugar for the meringue and add 1 tsp ground cinnamon with the vanilla.

Troubleshooting

Buttercream is soupy and will not thicken

The meringue or butter was too warm. Refrigerate the whole bowl for 15 to 20 minutes, then beat again with the paddle. Repeat if needed.

Buttercream looks curdled, like cottage cheese

It is too cold. Keep beating while you warm the outside of the bowl with a warm towel or a hair dryer for a few seconds at a time, or melt 2 tbsp of it and beat it back in.

Meringue will not whip to stiff peaks

Fat got into the whites, often a speck of yolk or a greasy bowl. Start again with clean equipment and fresh whites.

Storage

Refrigerate in an airtight container for up to 1 week or freeze for up to 3 months. Thaw in the refrigerator, let it come to room temperature for several hours, then re-beat with the paddle until silky. A frosted cake can sit at cool room temperature for a day.

Safety notes

Heating the whites to 71°C (160°F) follows the FDA's recommended temperature for dishes containing eggs. Contains egg and milk.