In the top of a double boiler, whisk together sugar and egg whites until combined. Cook, without stirring, over simmering water until a candy thermometer registers 140°. Immediately pour mixture into the work bowl of a heavy-duty stand mixer, and beat at high speed for 10 minutes.
Reduce speed to medium-low, and add butter, 2 tablespoons at a time, beating just until combined after each addition. Beat in syrup until combined. Store buttercream at room temperature until ready to use.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/dark-chocolate-cupcakes-cherry-buttercream-recipe/