This recipe is used in our Toffee Temptation Cake.
Combine egg whites and sugar in a heatproof mixer bowl set over pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (mixture should feel completely smooth when rubbed between your fingertips).
Attach the bowl to a mixer fitted with whisk attachment. Starting on low and gradually increasing to medium-high speed, whisk until stiff (but not dry) peaks form. Continue mixing until mixture is fluffy and glossy and completely cool (test by touching the bottom of the bowl), about 10 minutes.
With mixer on medium-low speed, add the butter a few tablespoons at a time, mixing well after each addition. Once all butter has been added, whisk in vanilla, then the bourbon. Keep buttercream at room temperature if using the same day; or store in a plastic airtight container in the refrigerator for up to 1 week, or in the freezer for up to 1 month.