White Mousseline Buttercream

This buttercream is made with egg whites instead of the more classic buttercream made with yolks. I have come to prefer it. It's a little lighter in both taste and texture. Buttercream frostings aren't overly sweet, have a rich, creamy consistency, and offer many options for flavoring.


1 cup White Sugar
¼ cup Water
cups Unsalted ButterSoftened to the consistency of mayonnaise
¼ tsp Cream of TartarScant
1 tbsp VanillaAdditional flavorings may be added as well, such as peppermint, or fruit purees
5   Egg WhitesPreferably, at room temperature


1 A free-standing mixer is helpful, though not required. However, some kind of electric mixer is a must.
2 Put the egg whites in a large mixing bowl. Have nearby the cream of tartar, softened butter, and vanilla. (A note about the butter: it should be about the same consistency as a frosting in order for it to mix in successfully)
3 Have a candy thermometer near the stove. Also near the stove, I usually have a heavy mug with water in it so that when the syrup is cooked I can quickly place the thermometer in a safe place. Also, for cooking the sugar syrup, you'll need a small pan with a cover.
4 The reason a free-standing mixer is helpful is that you need both to beat the egg whites and cook the sugar syrup simultaneously. Without a free standing mixer, you need to improvise by going back and forth a bit between beating the egg whites and cooking the syrup.
5 Start by getting the egg whites beating. Initially beat them briefly, until they look foamy. Then add the cream of tartar. With a free-standing mixer, continue mixing the whites on a medium setting. With a portable beater, beat the whites until they are just softly peaked.
6 Place the uncovered pan over medium heat. In the pan put the water and sugar. Stirring continuously until the sugar is completely dissolved, let the mixture come to a boil. Stop stirring, cover the pan, and let the steam "wash" down any sugar crystals which may have adhered to the sides of the pan. Such crystals can interfere with the creaminess of the finished product.
7 Uncover the pan and let the syrup continue to cook, without stirring. When the syrup just reaches 250 degrees according to the thermometer, quickly remove the pan from the heat and place the thermometer in the water-filled mug.
8 With the portable mixer, working as smoothly as possible, with the egg whites beginning to hold a pretty good peak, pour the syrup slowly into the egg whites, making sure not to hit the beaters (or the syrup will immediately harden onto the beaters). Continue to add the syrup with the beaters at high speed. This is cooking the egg whites as well as beating them. When all the syrup has been added, continue to beat the mixture until it is at room temperature.
9 With the free standing mixer, working as smoothly as possible, and with the mixer turned off, add just a little of the cooked syrup, making sure it falls free of the beaters and the sides of the mixing bowl (or the syrup will harden onto the beaters and sides of bowl). Immediately turn the mixer on to a very slow speed at first to incorporate the syrup with the egg whites (just for a second or so). Then turn the mixer onto a high speed for just five seconds. Keep repeating this process, adding a little more syrup each time. After all the syrup has been added, let the mixer continue to run until the mixture is at room temperature.
10 With both mixtures, you will find the volume increases greatly. The cooling takes quite a few minutes, at least ten or so.
11 With the mixer running, add the butter by tablespoons, only adding the next when the previous has been well incorporated into the mixture. This will change the consistency of the frosting somewhat. That's okay. It will still be delicious.
12 After the butter is completely incorporated, add the vanilla. The frosting is now ready to be used.
13 Note: I have made this frosting successfully with less butter, both with three quarters of a pound and a full pound. Of course this would then make a little less frosting.
14 I have added peppermint to this frosting for mint brownies. The frosting is also wonderful with added strawberry puree, about three-quarters of a cup. (See recipe for strawberry puree.) It's also very special with added hazelnut praline paste (about three-quarters of a cup) which I obtain from the King Arthur Flour Baking Company. The company has a store in Vermont and also sells through a catalog.
15 This recipe makes plenty of frosting to fill and frost an eight or nine inch layer cake.