Directions

1. Put egg yolks and 1/2 cup sugar into the bowl of an electric mixer fitted with the whisk attachment; beat on high speed until pale and thickened, 2 to 3 minutes.
2. Bring milk and vanilla to a boil in a medium saucepan. Remove from heat. Whisk about one-third of the liquid into yolk mixture. Pour mixture back into pan with remaining custard; whisk to combine. Cook over medium heat, stirring constantly, until mixture registers 185 degrees on an instant-read thermometer. Remove from heat; strain. Refrigerate until cool.
3. Put butter into the bowl of an electric mixer fitted with the paddle attachment; mix on medium-high speed until pale and fluffy. Mix in chilled custard.
4. Heat egg whites and remaining 1 cup sugar in the clean heatproof bowl of an electric mixer set over a pan of simmering water, whisking constantly, until sugar has dissolved. Attach bowl to mixer fitted with the whisk attachment; beat on high speed until stiff peaks form.
5. Add egg-white mixture to butter mixture; beat on medium-high speed until smooth. Refrigerate in an airtight container up to 3 days; bring to room temperature, and beat before using.