|heavy cream, hot|
|unsalted butter (65° to 75°F/19° to 23°C)|
|pure vanilla extract||.||.|
Have ready a
In a medium heavy saucepan, preferably nonstick, stir together the sugar, corn syrup, and water until all the sugar is moistened. Heat, stirring constantly, until the sugar dissolves and the syrup is bubbling. Stop stirring completely and let the syrup boil undisturbed until it turns a dark amber (380°F/193°C). Remove it immediately from the heat because the temperature will continue to rise, or remove it slightly before it reaches temperature, and just as soon as it reaches temperature, slowly pour in the hot cream. It will bubble up furiously.
Use a silicone spatula or wooden spoon to stir the mixture gently, scraping the thicker part that settles on the bottom. Return it to very low heat, continuing to stir gently for 1 minute, until the mixture is uniform in color and the caramel fully dissolved.
Remove it from the heat and gently stir in the butter until incorporated. The mixture will be a little streaky but becomes uniform once cooled and stirred.
Pour the caramel into the prepared glass measure and let it cool for 3 minutes. Gently stir in the vanilla. Refrigerate for about 45 minutes, gently stirring every 15 minutes until cool to the touch. (An instant-read thermometer should read 70° to 75°F/21° to 24°C.)
In the top of a double boiler over barely simmering water (do not let the bottom of the container touch the water), melt the white chocolate and butter, stirring often, until smooth and creamy.
Whisk the eggs lightly to break them up and then whisk them into the melted chocolate until incorporated. With a silicone spatula, continue stirring, being sure to scrape the mixture from the bottom of the container to prevent overcooking. Stir until an instant-read thermometer reads 160°F/71°C. The mixture will have thickened slightly. Remove it from the heat source. (If any lumps have formed, press them through a strainer.) Transfer the mixture into a bowl. Cover it tightly, and refrigerate for about 1 hour, stirring every 15 minutes until cool to the touch. (An instant-read thermometer should read 65° to 70°F/19° to 21°C.) To speed cooling, place the bowl in an ice water bath and stir often.
In the bowl of a stand mixer fitted with the whisk beater, beat the butter on medium-low speed until creamy, about 30 seconds.
Gradually beat the white chocolate custard base into the butter, scraping down the sides of the bowl as needed. Raise the speed to medium-high and beat for 2 minutes, until stiff peaks form when the beater is raised.
Cover and set aside for 1½ to 2 hours, or until the mixture is slightly thickened and spongy. It should be no warmer than 70°F/21°C. If necessary, place the bowl in an ice water bath for a few minutes and stir constantly. Beat on medium-high speed for 30 seconds until smooth, light, and creamy.