1. Pour 100ml (3 1/2fl oz) water into a saucepan and add 250g (8oz) caster sugar. Place the pan on the hob and stir until the sugar dissolves. Bring the mixture to the boil. Use a damp pastry brush to wash down any sugar crystals on the side of the pan. Boil the mixture rapidly, without stirring it, until it reaches 121°C. Have a large bowl of cold water ready for step 3.
2. When the syrup reaches over 100°C, whisk the egg whites until stiff, preferably using a table-top mixer. Gradually whisk in the remaining caster sugar.
3. As soon as the syrup reaches 121°C, remove the pan from the heat and plunge the base of the pan into the bowl of cold water, to prevent the syrup from getting any hotter. Only leave the pan in the water for a few seconds - if it's left too long, it will be too thick to pour.
4. With the food mixer on full speed, gradually pour in the syrup in a thin stream, taking care to pour it between the edge of the bowl and the whisk - if it's too close to the edge of the bowl, it will set there and won't get mixed in properly; if it's poured over the moving whisk, then it will splatter out of the bowl and make a mess. Continue whisking the mixture for about 8-10 minutes until the bowl feels just lukewarm. If the syrup starts to become too thick to pour, return the pan to the hob very briefly, for the heat to thin is slightly.
5. Gradually whisk in the butter. Then add the salt and vanilla extract. The meringue will collapse a little, and the mixure may look like it's curdled, but keep whisking it until it forms a smooth fluffy buttercream. Use paste colour to colour the buttercream to the desired colours.