|unsalted butter, preferably high fat, slightly softened but cool (65° to 68°F/19° to 20°C; see Note)|
|cream of tartar||.|
If using the stand mixer for the egg whites, attach the whisk beater. Beat the egg whites and cream of tartar on medium speed until foamy. Raise the speed to medium-high and beat until soft peaks form when the beater is raised. Gradually beat in the remaining
Increase the heat under the sugar syrup and continue to boil the syrup for a few minutes until an instant-read thermometer reads 248° to 250°F/120°C. Immediately transfer the syrup to the glass measure to stop the cooking.
If using a stand mixer, with the mixer off to keep the syrup from spinning onto the sides of the bowl, add it to the egg whites. Begin by pouring in a small amount of syrup. Immediately beat on high speed for 5 seconds. Add the remaining syrup the same way in three parts. For the last addition, use a silicone spatula to remove the syrup clinging to the glass measure and scrape it off against the whisk beater. If the syrup has hardened before most of it has been poured, soften it to pouring consistency for a few seconds in the microwave.
If using a handheld mixer, beat the syrup into the egg whites in a steady stream. Do not let the syrup fall on the beaters or they will spin it onto the sides of the bowl.
Lower the speed to medium and continue beating for up to 2 minutes. Refrigerate the meringue for 5 to 10 minutes, or until an instant-read thermometer reads 70°F/21°C. Whisk after the first 5 minutes to test and equalize the temperature.
Set the mixer bowl containing the butter in the stand mixer and attach the whisk beater (no need to wash it). Beat on medium-high speed for about 3 minutes, or until the butter lightens in color and is no warmer than 70°F/21°C.
Scrape the meringue into the butter and beat on medium speed until smooth and creamy. Beat for about 2 minutes, scraping down the sides of the bowl as necessary. At first it will look slightly curdled. If it starts watering out, check the temperature. The mixture should feel cool and be no lower than 65°F/19°C, no higher than 70°F/21°C. If it is too warm, place the bowl in an ice water bath and stir gently to chill it down before continuing to whisk. If it is too cool, suspend the bowl over a pan of simmering water (do not let the bottom of the bowl touch the water) and heat very briefly, stirring vigorously when the mixture just starts to melt slightly at the edges. To stop the warming, dip the bottom of the bowl in an ice water bath for a few seconds to cool it. Remove the bowl from the ice water and beat the mousseline by hand until smooth.
Beat in the raspberry sauce until uniformly mixed. The mousseline becomes spongy and fluffy on standing. If it was not used right away, whisk it lightly by hand to maintain a silky texture before applying it to the cupcakes. If it was refrigerated let it come to room temperature (at least 70°F/21°C) before whisking; this prevents it from breaking down.
Copyright © 2014 by Cordon Rose, LLC. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.