- Place the egg yolks in large bowl. Set aside.
- In a saucepan, bring the water and sugar to a boil. Simmer until a candy thermometer reads 117 °C (242 °F), about 5 minutes.
- While beating with and electric mixer at medium speed, drizzle the hot syrup over the egg yolks, avoiding the beaters of the mixer. Beat until completely cooled, about 15 minutes.
- Add the butter, 1 or 2 pieces at a time, beating on medium speed until the buttercream is light and fluffy. Add the vanilla and stir to combine.
- This buttercream can be refrigerated for 3 to 4 days or frozen for up to four months. Just bring it to room temperature and beat again before frosting the cake.
It is important that the egg mixture is completely cooled before adding the butter. Do not worry if, after adding about half the butter, the mixture seems to curdle. Everything should return to normal by adding the remaining butter.