"French buttercream is prepared in the same way as Italian meringue-based buttercream, except egg yolks (some versions use whole eggs or a combination of the two) are used in place of the egg whites—a hot sugar syrup which has reached the soft-ball stage is beaten into the egg yolks which have been beaten until they are thick and pale yellow. The syrup and egg yolk mixture is further whipped until it has formed a light foam and has cooled. Butter and flavorings (extracts, oils, or juices) are then whipped in. This icing is very rich, smooth, and light. French buttercream tends to melt faster than other buttercreams due to the high content of fat from the egg yolks and butter. This type of buttercream is best suited for use as a filling or an icing, but not for decorations." - Wikipedia
The following recipe is truly the best that I've used and is found in the CIA textbook Baking and Pastry: Mastering the Art and Craft. The addition of sea salt and notes in [ ] are from my experience and not in the original recipe. I strictly use this type of buttercream as a filling, for example in rolled cakes.
Makes 3 lb (1.36 kg) - [mixes well in a 6-qt bowl of a stand mixer]
- 8 oz (227 g) whole eggs [at room temperature]
- 8 oz (227 g) egg yolks [at room temperature]
- 1 lb 2 oz (510 g) granulated sugar
- 4 fl oz (120 ml) water
- 1 lb 8 oz (680 g) unsalted butter, cut into medium chunks, soft
- 1 tsp sea salt
- 2 tsp (10 ml) vanilla extract [or vanilla bean paste]
- Whip the eggs and yolks in an electric mixer fitted with a wire whip attachment on high speed until light and fluffy, about 5 minutes.
- Combine the sugar and water in a heavy-bottomed saucepan and bring to a boil, stirring to dissolve the sugar. Continue cooking, without stirring, until the mixture reaches 235 degrees F/113 degrees C.
- Slowly pour the hot sugar syrup into the eggs while whipping on medium speed. [Add sea salt and] continue to whip until cool.
- Gradually add the butter, beating until incorporated after each addition and scraping down the sides of the bowl as necessary. Blend in the vanilla [or vanilla bean paste].
- Store, covered, under refrigeration until ready to use. [Double wrapping in plastic film is the easiest and best way to store your buttercream. And double wrapping prevents food odors from seeping into your frosting. An off tasting buttercream is not good eats.]