Mint Buttercream

Makes 3 1/2 cups
2 large eggs, separated
1/2 cup sugar
2/3 cup milk
1/3 teaspoon pure vanilla extract
2/3 cup coarsely chopped fresh mint leaves
1 pound (4 sticks) unsalted butter, room temperature
1/4 teaspoon pure peppermint extract
Directions
Whisk together yolks and 1/4 cup sugar with a mixer on high speed until pale and thickened, 2 to 3 minutes.
Prepare an ice-water bath. Bring milk, vanilla, and mint leaves just to a boil in a medium saucepan. Remove from heat. Whisk about 1/3 of milk mixture into yolk mixture. Pour yolk mixture back into pan with remaining milk mixture, and whisk to combine. Place over medium heat, and cook, stirring constantly, until custard registers 185 degrees on an instant-read thermometer. Remove from heat; strain through a fine sieve into a bowl. Set bowl in ice-water bath, and stir until cool.
Beat butter with a mixer on medium-high speed until pale and fluffy. Add chilled custard to butter, and beat until incorporated.
Heat whites and remaining 1/4 cup sugar in a heatproof mixer bowl set over a pan of simmering water, whisking constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips). Attach bowl to mixer fitted with whisk attachment. Starting on low, and gradually increasing to medium-high speed, whisk until stiff peaks form.
Add egg-white mixture to butter mixture, switch to paddle attachment, and beat on medium-high speed until smooth. Beat in peppermint extract. Use immediately, or cover and refrigerate for up to 3 days or freeze up to 3 months. (Before using, bring to room temperature, and beat on low speed until smooth.)