This is a lighter type of buttercream. Due to the addition of egg whites and cream of tartar, besides being lighter, it is more stable in hot climates also.
These quantities are enough to make swirls in about 40 regular sized cupcakes.
SWISS MERINGUE BUTTERCREAM
5 large egg whites
225g caster sugar
pinch of salt
½ tsp cream of tartar
400g unsalted butter, cut into tablespoons, room temperature
1 ½ teaspoons pure vanilla extract
Combine egg whites, sugar, and salt in the heatproof bowl of a stand mixer set over a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips).
Add cream of tartar and attach the bowl to the mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, whisk until stiff (but not dry) peaks form. Continue mixing until the mixture is fluffy and glossy, and completely cool (test by touching the bottom of the bowl) – this will take about 10 minutes.
With mixer on medium-low speed, add butter a few tablespoons at a time, mixing well after each addition. Once all butter has been added, whisk in vanilla. Switch to the paddle attachment, and continue beating on low speed until all air bubbles are eliminated – about 2 minutes. Scrape down sides of bowl with a flexible spatula, and continue beating until the frosting is completely smooth. Keep buttercream at room temperature if using the same day.
To tint buttercream, reserve some for toning down the colour, if necessary. Add gel-paste food colouring, a drop at a time (or use a toothpick or skewer to add food colouring a dab at a time) to the remaining buttercream. You can use a single shade of food colouring or experiment by mixing two or more. Blend after each addition with the mixer (use a paddle attachment) or a flexible spatula, until desired shade is achieved. Avoid adding too much food colouring too soon, as the hue will intensify with continued stirring; if necessary, you can tone down the shade by mixing in some of the reserved untinted buttercream.