This is similar to Italian Meringue Buttercream, which uses egg whites exclusively and is very light and fluffy, and Egg Yolk French Buttercream, which uses only egg yolks and is very rich. This recipe for Whole Egg French Buttercream uses whole eggs and is in the middle in terms of color and richness.
Low FODMAP Serving Size Info: Makes about 4 cups (960 ml); keep serving size to about 1/4 to 1/2 cup (60 ml to 120 ml); 8 to 16 servings
Whisk together the sugar and water in a deep heavy saucepan and bring to a boil over medium-high heat. Cook this sugar syrup, undisturbed, until it reaches 240°F/116°C.
Meanwhile, while the sugar syrup is boiling, whip the eggs and salt in the bowl of a stand mixer with a balloon whisk/whip attachment on medium speed until well beaten.
Once the sugar syrup is ready, and with the mixer running, very slowly pour the hot syrup onto the egg mixture, avoiding the beater and the sides of the bowl. Continue to beat on high until buttercream is completely cool, which could take several minutes.
Once the buttercream is cool to the touch, add the butter, a piece at a time, beating on a medium-high speed all the while. Keep beating until all of the butter is added and the buttercream is smooth and creamy. Beat in vanilla. Buttercream is ready to use. Buttercream is best if used immediately. It can also be refrigerated in an airtight container in refrigerator or freezer. It MUST be brought back to a warm room temperature and re-beaten before using. If re-beating after chilling, use the flat paddle attachment.