Mint Buttercream

Preparation

1. Step 1

   In the bowl of a standing electric mixer fitted with the whisk attachment, mix egg yolks and 1/4 cup sugar on high speed until pale and thick, 2 to 3 minutes.

   Step 2

   Prepare an ice-water bath. Bring milk, vanilla, and mint leaves just to a boil in a medium saucepan. Remove from heat. Whisk about one third of milk mixture into yolk mixture (this is called tempering, which keeps the yolks from curdling). Pour yolk mixture into pan with remaining milk mixture, and whisk to combine. Clip candy thermometer to side of pan. Cook over medium heat, stirring constantly, until mixture registers 185°F. Remove from heat; strain through a fine sieve into a heatproof bowl (discard solids). Set bowl in ice bath, stirring mixture until cool.

   Step 3

   In another bowl of an electric mixture fitted with the paddle attachment, cream butter on medium-high speed until pale and fluffy. Beat in chilled egg-yolk mixture.

   Step 4

   Heat egg whites and remaining 1/4 cup sugar in the clean heatproof bowl of an electric mixer 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 the mixer fitted with the clean whisk attachment; starting on low speed and gradually increasing to medium-high speed, whisk until stiff (but not dry) peaks form and mixture is completely cool (test by touching bottom of bowl), about 10 minutes.

   Step 5

   Add egg-white mixture to butter mixture; switch to the paddle attachment, and beat on medium-high speed until smooth. Beat in peppermint extract. Keep buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 3 months. Before using, bring to room temperature, and beat with the paddle attachment on low speed until smooth again, about 5 minutes.