- Place the egg in large bowl. Set aside.
- In a saucepan, bring the sugar, syrup, and water to a boil. Simmer until the candy thermometer reads 117 °C (242 °F), about 3 to 4 minutes.
- While beating with an electric mixer on medium speed, drizzle the hot syrup into the egg, avoiding the beaters. Beat until completely cooled, about 15 minutes.
- Add the butter, 1 or 2 pieces at a time, beating until the buttercream is light and fluffy (see note). With a spatula, scrape the bottom of the bowl several times during this stage.
- Add the vanilla and colour the buttercream, if desired.
If the buttercream seems too soft, refrigerate it for a few minutes, and then beat it again. Corn syrup prevents the buttercream from crystallizing.
A classic rich buttercream with an unbeatable texture.