Rose's Baking Basics

Mousseline

Makes 440 grams/2¼ cups mousseline; double the recipe for a 9 inch two-layer cake or 13 by 9 inch sheet cake

This is the queen of buttercreams. It has the lightest texture and holds up best at warmer temperatures, but it is difficult to make in hot and humid weather. The butter needs to be softened but cool, and high-fat butter works best. An accurate instant-read thermometer is imperative.

unsalted butter, preferably high-fat	227 grams	16 tablespoons (2 sticks)
2½ large egg whites	75 grams	¼ cup plus 1 tablespoon (74 ml)
cream of tartar	.	½ teaspoon
sugar, preferably superfine	100 grams	½ cup, divided
water	30 grams	2 tablespoons (30 ml)
pure vanilla extract	.	½ teaspoon (2.5 ml)
liqueur of your choice (optional)	42 to 90 grams	3 to 6 tablespoons (45 to 90 ml)

Mise en Place

▪ Thirty minutes to 1 hour ahead, set the butter on the counter at cool room temperature (65º to 70ºF/19º to 21ºC). The butter needs to be about 65ºF/19ºC.

▪ Thirty minutes to 1 hour ahead, into a medium bowl, weigh or measure the egg whites and add the cream of tartar. Cover with plastic wrap.

▪ Have ready a 1 cup glass measure with a spout (not coated with nonstick cooking spray) by the range.

Make the Mousseline

1. In the bowl of a stand mixer fitted with the flat beater, beat the butter on medium-high speed until creamy, about 1 minute. Set it aside in a cool place (no higher than 70ºF/21ºC).

2. In a small heavy saucepan, preferably nonstick and with a spout, stir together 75 grams/¼ cup plus 2 tablespoons of the sugar and the water until all of the sugar is moistened. Heat on medium, stirring constantly, until the sugar dissolves and the mixture is bubbling. Stop stirring and reduce the heat to low. (On an electric range remove the pan from the heat.)

3. With a handheld mixer, beat the egg whites on medium-low speed until foamy. Raise the speed to high and beat until soft peaks form when the beater is raised. Gradually beat in the remaining 25 grams/2 tablespoons sugar, until stiff peaks form when the beater is raised slowly.

4. Increase the heat under the sugar syrup and continue to boil the syrup for a few minutes until an instant-read thermometer reads 248º to 250ºF/120ºC. Immediately pour the syrup into the glass measure to stop the cooking.

5. Beat the syrup into the egg whites in a steady stream. Don’t allow the syrup to fall on the beaters or they will spin it onto the sides of the bowl. Use a silicone spatula to remove the syrup clinging to the measure.

6. Lower the speed to medium and continue beating for up to 2 minutes. Refrigerate the meringue for 5 to 10 minutes, until 70ºF/21ºC. Whisk it after the first 5 minutes to test and equalize the temperature.

7. Set the mixer bowl containing the butter on the stand and attach the whisk beater. Beat on medium-high speed for about 3 minutes or until it lightens in color and is no warmer than 70ºF/21ºC.

8. Scrape the meringue into the butter and beat on medium speed until smooth and creamy, about 2 minutes, scraping down the sides of the bowl as necessary. At first the mixture will look slightly curdled. If it starts to separate and become watery, check the temperature: It should feel cool, but be no lower than 68ºF/20ºC and no higher than 70ºF/21ºC. If too warm, set it in a bowl of ice water, stirring gently to chill it down before continuing to whisk. If too cool, suspend the bowl over a pan of simmering water (do not let the bottom of the bowl touch the water) and heat for just a few seconds, stirring vigorously when the mixture just starts to melt slightly at the edges. Dip the bottom of the bowl in a larger bowl of ice water for a few seconds to cool it. Remove the bowl from the ice water and beat the buttercream by hand until smooth. If the mixture breaks down and will not come together, it can still be rescued. See Solutions for Possible Problems.

9. Gradually beat in the vanilla and the liqueur, if using.

Store Airtight: room temperature, 1 day; refrigerated, 3 days; frozen, 2 months.

Make This Recipe Your Own

See Additions to Neoclassic Buttercream. Use a liqueur that is compatible with your choice of additions.

Baking Pearls

It is best to avoid making meringue on humid days.

The mixer bowl and beater must be entirely free of any fat, which includes oil and egg yolk.

If doubling the recipe, it’s still fine to use a stand mixer for the egg whites, but be sure to avoid letting the syrup hit the beaters so that it doesn’t spin it onto the sides of the bowl.

The mousseline becomes spongy and fluffy on standing, which is lovely once on the cake. If you don’t use it right away, whisk it lightly by hand to maintain a silky texture before applying it to the cake. Do not, however, rebeat chilled mousseline until it has reached 70°F/21°C; otherwise it may break down. If this happens, see Solutions for Possible Problems.

Making Mousseline