Beat eggs on medium speed until very foamy, gradually add sugar while beating. Place the bowl over a barely simmering pot of water. Beat eggs for about 5 minutes until eggs are very thick and creamy and very warm (not hot). Remove from heat and cool down custard by continually beating. Set aside

Cream the butter until pale in color and very creamy, this will take about 10 minutes. Add vanilla flavouring into the butter and beat.

Very slowly add room temperature custard to butter mixture. Continue beating and scraping sides of bowl until buttercream is light and fluffy.