Beat the egg yolks for a few minutes until frothy.
In a thick bottomed saucepan , heat the sugar and water without stirring , until the sugar dissolves and it reaches 238 F/ soft ball stage. That is, a drop of syrup into cold water will form a soft ball that you can roll between your fingers.
Immediately take the sugar syrup off the heat , and drizzle into the egg yolks gently , continuously and slowly , while whipping them.
Once all the syrup has been added, keep beating the mixture until the bowl is cool to touch.
Add the butter, one cube at a time , beating for a few seconds after each addition.
Beat until the buttercream is light and fluffy( a few minutes) .
Add vanilla / flavouring and beat until incorporated.