Frosting - Mousseline - Strawberry Recipe

Ingredients

Volume Weight
unsalted butter, pref hi fat softened but cool -65 degrees 2 1/2 sticks /10 oz / 284 grams
3 large egg whites, room temp (3 fl oz)/ 1/4 cup plus 2 tbs/3.2 oz /90 g
sugar 2/3 cup, divided /4.6 oz/ 132 g
water 3 tbs /1.5 oz/44 g
cream of tartar 1/4 plus 1/8 tsp - -
tart strawberry butter- 2/3 cup plus 1/2 cup divided/6.7 oz plus 5 oz/ /190 g +142g
vanilla 1/2 tsp - -
3 drops food color (opt)

Preparation

Step 1

In bowl of stand mixer fitted w/ flat beater beat butter on med high til creamy,, abt 1 min
Set aside in cool place, no higher than 70 degrees
Pour egg whites into bowl of stand mixer
Have ready a 1 cup or larger heatproof glass measure
In small heavy saucepan,preferably nonstick, stir together all but 3tbs of the sugar & the water until all of the sugar is moistened.
Heat on med, stirring constantly til sugar dissolves & the mixture is bubbling
Stop stirring & reduce the heat to low (if using elec stove remove pan from heat)
Attach whisk beater to mixer, beat on med speed til foamy
With mixer off, add cream of tartar
Raise speed to med high & beat til soft peaks form when beater is lifted
Gradually beat in the remaining 3 tbs sugar til stiff peaks
Inc the heat under the sugar syrup & continue to boil for a few mins til instant read thermometer registers 248 to 250 (firm ball stage)
Immediately transfer the syrup to glass measure to stop the cooking
With mixer off to keep from spinning onto sides of bowl, add syrup to the egg whites. Begin by pouring in a small amt of syrup, immediately beat on high for 5 seconds. Add the remaining syrup the same way in 3 parts. For the last addition use a silicon scraper to remove the syrup clinging to the syrup & scrape it off against the whisk beater. If syrup has hardened before most of it has been poured, soften it to pouring consistency for a few secs in the microwave
Lower the speed to medium & continue beating for up to 2 mins
Refrigerate the meringue for 5 - 10 mins or til temp registers 70 degrees
Whisk after the first 5 mins to test & equalize the temp
Set the mixer bowl containing the butter in mixer & attach whisk (no need to wash it) Beat on med high speed for abt 3 mins or til the butter lightens in color & is no warmer than 70 degrees
Scrape the meringue into the butter & beat on med til smooth & creamy. Beat for abt 2 mins, scraping down bowl as nec
At first the mixture will look curdled
If it starts watering out check the temp The mixture should feel cool & be no lower than 65, no higher than 70
If it is too warm set the bowl in a larger bowl of ice water & stir gently to chill it down before continuing to whisk
If it is too cool, suspend the bowl over a pan of simmering water (dont let the bottom of the bowl touch the water)& heat very briefly, stirring vigorously when the mixture just starts to melt slightly at the edges. To stop the warming dip 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 & beat the frosting by hand til smooth
Beat in 2/3 cup of strawberry butter, vanilla & food color if using til uniformly mixed
The mousseline becomes spongy & fluffy on standing. If you dont use it right away , whisk it lightly by hand to maintain a silky texture before applying it to the cake. chilled mousseline However do not rebeat chilled
mousseline til it has reached 70 degrees-this will prevent it from breaking down
Sandwich each cake layers w/ mousseline & a layer of strawberry butter