- 3 large eggs, separated
- 3/4 cup sugar
- 1 cup whole milk
- 3 cups (6 sticks) unsalted butter, at room temperature
- 3/4 cup pistachio paste
Combine the egg yolks and 1/4 cup sugar in a mixer bowl; whisk on high speed until pale and thickened, 2 to 3 minutes.
Bring the milk to a boil in a medium saucepan; remove from heat. Whisk about one-third of the milk into the yolk mixture. Pour the mixture back into the pan with the remaining milk; whisk to combine. Cook over medium heat, stirring constantly, until the mixture registers 185 degrees on an instant-read thermometer. Remove from heat; strain. Refrigerate until cool.
Put the butter into a clean mixer bowl; beat on medium-high speed until pale and fluffy. Mix in the chilled custard and the pistachio paste; set aside.
Heat the egg whites and remaining 1/2 cup sugar in a clean heatproof mixer bowl set over a pan of simmering water, whisking constantly, until sugar has dissolved. Attach bowl to mixer, and whisk on high speed until stiff peaks form.
Add the egg-white mixture to the butter mixture; beat on medium-high speed until smooth.