Sweet, tart, creamy, and crunchy, this sophisticated sundae is a riot of flavor and texture. Both the lemon curd and the vanilla ice cream showcase the richness of egg yolks.
Make the lemon curd: Cook egg yolks, sugar, and lemon juice and zest in a medium saucepan over medium-high heat, whisking constantly, until bubbling in center, about 6 minutes. Remove from heat, and whisk in butter, a few tablespoons at a time.
Immediately pass through a fine sieve into a bowl. Place plastic wrap directly on surface of curd to prevent skin from forming. Let cool; refrigerate at least 2 hours and up to 3 days.
Make the pistachios: Coat a rimmed baking sheet with cooking spray. Bring sugar and water to a boil over medium-high heat, stirring to dissolve sugar. Cook until a candy thermometer registers 238 degrees, about 6 minutes. Add nuts and salt, then cook, stirring, until sugar crystallizes, about 1 1/2 minutes. Pour mixture onto baking sheet, and let cool 30 minutes. Roughly chop nuts. Nuts can be stored up to 1 week.
To serve: Scoop ice cream into dishes. Top with curd and whipped cream, then sprinkle with nuts.