40g egg white
30g caster sugar
100g unsalted butter (cold)
1. Pour sugar and egg white into a heatproof pan and place it over a saucepan of simmering water.
2. Whisk constantly until sugar has dissolved and the mixture is warm to the touch.
3. Pour the egg white into a mixing bowl with the whisk attachment and beat on medium-high speed until stiff peaks (but not dry).
4. In another mixing bowl whisk cold butter until pale and creamy.
5. Add meringue some at a time into creamed butter and whisk until well blended then repeat procedure. (Make sure that the whipped butter and meringue are well mix together to avoid buttercream from separating.)
6. Keep buttercream in an air tight container and store in the fridge.