5 large egg whites
1 c. plus 2 Tbsp. sugar
pinch of salt
1 lb. (4 sticks) unsalted butter at room temperature
1/2 c. Vanilla White Balsamic (or flavored balsamic of your choice)
*Note: dark balsamic vinegar reduces at a much faster rate than white balsamic
First, reduce the balsamic vinegar by approximately 1/3 and let the reduction cool to room temperature. The easiest way to do this is to place the balsamic in a heat-proof glass dish in the microwave. 1/2 cup will reduce in approximately 5 minutes; check it at 3 minutes, then continue if needed.
Combine the egg whites, sugar, and salt in a heat-proof mixing bowl over simmering water. Heat while whisking frequently until the mixture reaches 160 degrees and the sugar has dissolved. Remove the bowl from the simmering water and beat on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, about 8 minutes. Reduce the mixer speed to medium and add the butter, 2 tablespoons at a time, waiting for each addition to be incorporated before adding more. The frosting will start to look curdled; continue beating on medium-high speed until thick and smooth looking again, about 3 to 5 minutes. Stir in a few teaspoons of the balsamic reduction and mix just enough to incorporate. Taste, and add more of the reduction if a stronger flavor is desired.
Note 1: When reducing balsamic, 1/2 a cup may reduce more quickly. Try heating in 1 minute increments and make sure you don't let it get too thick or it will turn into candy when it has cooled.
Note 2: Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate for up to 3 days, or freeze for up to 1 month. Before using, bring to room temperature and beat on low speed until smooth again, about 5 minutes.