Whisk yolks and eggs in a medium bowl to blend. Combine lemon juice, butter, and sugar in large metal bowl. Set bowl over saucepan of simmering water; whisk until butter melts and sugar dissolves. Gradually whisk 1/3 of hot butter mixture into eggs, whisk mixture back into bowl with remaining butter mixture set over simmering water. Whisk constantly until custard thickens and thermometer inserted into mixture registers 180°F, about 5 minutes (do not boil). (If you don’t have a candy thermometer, wait until the mixture thickens and coats the back of a wooden spoon.) Pour lemon curd through strainer set over medium bowl. Place plastic wrap directly on surface of lemon curd; chill overnight. (Can be made up to 2 days ahead. Keep refrigerated.)
Finely grind hazelnuts in processor. Using beat butter, powdered sugar, and lemon peel in a medium bowl until well blended. Add yolk and beat until smooth. Beat in hazelnuts, then flour, and salt. Gather dough into ball; flatten into disk.
Preheat oven to 350°F. Press crust onto bottom and up sides of a 9-inch-diameter tart pan or spring-form pan with removable bottom. Chill 20 minutes. Bake crust until golden brown and cooked through, pressing with fork if crust bubbles, about 25 minutes. Cool crust in pan on rack. (Can be prepared 1 day ahead. Cover and store at room temperature.)
Make Lemon Cloud
Using electric mixer, beat whipping cream, sugar, lemon peel, and lemon extract in large bowl until stiff peaks form.
Spoon half of lemon curd and half of lemon cream into another large bowl. Using small rubber spatula, gently fold curd and lemon cream together, creating marble effect and being careful not to over mix to retain marbling. Spoon marbled mixture in dollops into cooled crust. Using tip of knife or small rubber spatula, swirl and pull mixture upwards in peaks. Repeat with remaining curd and lemon cream mixture. Refrigerate tart at least 1 hour and up to 4 hours. Remove pan sides. Place tart on platter. Serve with salted caramel sauce.
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