Yields about 400g (2 cups) of buttercream. Enough to frost 1 cake or (not very generously) about 10 cupcakes. If you want to generously frost a cake, I advise you make 2 batches.
Ingredients
5 egg yolks
pinch of salt
100g (or ½ cup) granulated sugar
60ml (or ¼ cup) water
250g (or 1 cup + 5 teaspoons) unsalted butter, at room temperature
Instructions
First make a paté au bombe. To a small saucepan, add the sugar and water. Heat over low heat, stirring to dissolve the sugar, then crank up the heat, stop stirring and clip on a sugar thermometer. In the meantime, combine the egg yolks and salt in a heatproof bowl and mix until frothy. Once the sugar syrup reaches 114°C/238°F, remove it from the heat and slowly drizzle in the egg yolks, mixing continuously. Be sure to pour the syrup down the sides of the bowl, away from the whisk attachments, so that the syrup won't splatter. Once all the syrup has been added, raise the mixer's speed to high and beat until the mixture cools to body temperature and the paté au bombe has thickened and lightened in color.
When the paté au bombe is cool enough, start adding the butter, 1 tablespoon at a time, waiting until each tablespoon of butter is incorporated before adding the next. Once all the butter has been added, add the vanilla and mix to incorporate. Use immediately or cover with plastic wrap and place in the fridge until needed. Store in the fridge for up to 1 week or for up to 2 months in the freezer in a freeze-proof container or bag.
To use buttercream that has been refrigerated, allow to come to room temperature (this takes about 3 hours in a warm kitchen) or microwave in 30 second intervals, stirring in between each interval, until it's soft. Then mix through until it's spreadable again. Thaw frozen buttercream in the fridge overnight, then allow to come to room temperature before use and, once at room temperature, mix briefly until smooth.
Serve at room temperature. If you've assembled a cake or decorated cupcakes, allow cake or cupcakes to come to room temperature before serving (about 3 hours in a warm kitchen).
Recipe by The Tough Cookie at https://thetoughcookie.com/2013/12/01/battle-of-the-buttercreams-part-iii-french-buttercream/