makes 3 cups buttercream; recipe adapted from Julia Child and Nancy Kux of The San Francisco Baker's Dozen and The Baker's Dozen Cookbook
Whisk egg whites until foamy, using whisk attachment on a stand mixer. Sprinkle in the cream of tartar and a pinch of salt, then gradually increase speed to high and whisk until egg whites hold soft peaks. Turn the speed to low as you make the sugar syrup.
Combine the sugar and water in a small saucepan and set on the stovetop over medium heat. Bring to boil and cook, without stirring, until sugar is dissolved and temperature reaches soft-ball stage on a candy thermometer (225°–230°F at 5000ft. elevation, 235°–240°F sea level). Add the blackberry syrup and bring back to boil.
With mixer running on low, slowly drizzle the boiling sugar syrup into the egg whites, aiming for the space between the whisk and the bowl. When the sugar syrup has all been added, increase mixer speed to medium-high, and beat until cool and egg whites form "stiff, shining, upstanding peaks" (thanks Julia Child!), about 8 minutes.
With the mixer running on medium, add the butter, one tablespoon at a time, beating after each addition until it is absorbed into the meringue. Use to frost cupcakes same day, or refrigerate for up to 1 week. Freeze buttercream, or any leftovers, in a freezer bag for up to 3 months.