Combine the egg whites and sugar in a medium sized heatproof bowl. Set the bowl over a pan of simmering water, making sure the water does not touch the bottom of the bowl, gently heat the egg mixture, whisking continuously, until the sugar has dissolved. Remove the egg white mixture from the heat and using an electric whisk attachment beat the mixture until the meringue holds stiff peaks and has cooled to room temperature. Add the cubed butter, one cube at a time mixing after each addition. The mix may look like it is separating, but continue beating the mixture and the buttercream will come together. Pipe onto cold cupcakes or use how you want.