This buttercream can be used for different pastries... it's yummy that you will want to put it on everything!
2 cups sugar, divided
3/4 cup water
6 large egg whites at room temperature 30 minutes
2 tablspoons plus 1 teaspoon instant-espresso powder
1 tablspoon pure vanilla extract
1/2 teaspoon cream of tartar
1/4 teaspoon salt
6 sticks (1 1/2 pounds) unsalted butter, cut into tablespoon pieces and softened
Equipment: a candy thermometer; a stand mixer fitted with whisk attachment (I used a handheld and almost sprained my hand, but it was worth the near hospitalization)
Bring 1 3/4 cups sugar and water to a boil in a 3-quart heavy saucepan over medium heat, stirring until sugar has dissolved, then wash down any sugar crystals from side of pan with a pastry brush dipped in cold water. Boil, without stirring, until it registers 220 to 225°F, 15 to 20 minutes.
At this point, while continuing to boil syrup, beat whites with espresso powder, vanilla, cream of tartar, and salt in mixer at medium speed until they just hold soft peaks. Add remaining 1/4 cup sugar, 1 tablespoon at a time, beating, and beat until whites just hold stiff peaks.
When syrup reaches soft-ball stage (238 to 242°F), immediately pour syrup in a slow stream down side of bowl into whites (avoid beaters) while beating at high speed. Beat until completely cool, 25 to 30 minutes. With mixer at medium speed, add butter 1 tablespoon at a time, beating well after each addition (see cooks’ note, below) and until buttercream is smooth. (Mixture may look curdled before all butter is added but will come together at end.)
If buttercream looks soupy after some butter is added, meringue is too warm: Chill bottom of bowl in an ice bath for a few seconds before continuing to beat in remaining butter.
Buttercreams can be made 1 week ahead and chilled or 1 month ahead and frozen. Bring to room temperature (do not use a microwave), about 2 hours, and beat with an electric mixer until spreadable.
The egg whites in this recipe are not fully cooked