Whisk egg whites, sugar, and salt in the heatproof bowl of an electric mixer set over a pan of simmering water until sugar has dissolved and mixture registers 160 degrees, about 3 minutes.
Attach bowl to mixer fitted with the whisk attachment. Beat on medium-high speed until cooled, about 10 minutes. Reduce speed to medium. Add butter; beat until pale and fluffy. Mix in cream of coconut and extract. Buttercream can be refrigerated in an airtight container up to 3 days; beat before using.
When Martha used this buttercream to frost the Coconut Column Cake, she used 2 1/2 cups sugar.