Make the curd: Whisk the sugar, lemon zest, lemon juice and salt in a medium saucepan. Whisk the whole eggs and yolks in a small bowl, then whisk into the lemon mixture. Cook the mixture over medium heat, whisking constantly, until it's thick like pudding, 6 to 8 minutes. Remove from the heat and whisk in the butter a few pieces at a time until incorporated. Strain the mixture through a fine-mesh sieve into a small bowl, pushing it through with a rubber spatula. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until completely set, at least 4 hours and up to 5 days.
Make the tart: Preheat the oven to 350 degrees F. Break the graham crackers into pieces, then pulse in a food processor with the sugar until finely ground; drizzle in the melted butter and pulse to combine.
Press the crumb mixture into the bottom and about 1 inch up the side of a 9-inch springform pan. Bake until golden, about 10 minutes, then transfer to a rack and let cool completely.
Pour the curd into the prepared crust and gently press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until set, at least 4 hours. Remove the springform ring and transfer the tart to a platter. Top with whipped cream.
Photograph by Levi Brown