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
Pour the bag of frozen blackberries into a bowl and let thaw at room temperature for about 45 minutes. Strain the berries from the juice (enjoy the berries for another purpose)—you should have about 3 tablespoons of juice. Put juice in a small saucepan, add equal parts granulated sugar, bring to a boil on stovetop, lower heat to medium, and simmer a couple minutes until sugar is dissolved. Remove from heat and let cool.
Whisk egg whites until foamy using a stand mixer fitted with whisk attachment. Sprinkle in the cream of tartar, then gradually increase speed to high and whisk until egg whites hold soft peaks.
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 dissolves and temperature reaches “soft ball” stage on a candy thermometer (225F° in Bozeman). Add the blackberry syrup and a tiny pinch of salt; bring back to boil.
With mixer running at low speed, drizzle the boiling sugar syrup into the egg whites, then increase mixer speed to medium, and beat until cool and egg whites form “stiff, shining, upstanding peaks” (thanks, Julia Child!), about 6 minutes.
Keep the mixer running on medium speed and add the butter, 1 tablespoon at a time, beating after each addition until it is absorbed into the meringue. Use to frost cupcakes (or a cake!)
Baker’s Notes: The temperature of the butter is very important when making buttercream. It should be at warm-room temperature, about 70°F. If you see little dots of butter in your buttercream, your butter is too cool, so you need to warm things up—heat some of the remaining butter in the microwave for 5 seconds or warm the bottom of the bowl over the stovetop until some of the buttercream starts to melt—then back to the mixer. If your buttercream begins to look loose and curdled, it is too warm, so set your bowl in the refrigerator for a few minutes to cool, then proceed with mixing. Whatever you do, do not panic. You can fix most buttercream issues by playing with temperature, which is what bakers do when they make buttercream in advance and store it in the refrigerator or freezer.
Jam variation: Cut sugar in half and use equivalent amount of fruit jam (I've tested strawberry, but any high quality seedless jam should work); add jam to the sugar syrup after it has reached softball stage, stir to combine, and bring back to boil before drizzling into the meringue.