Caramel Pudding Tart with Almond Shortbread Crust

Butter 9-inch-diameter tart pan with removable bottom. Combine almonds and 1/4 cup sugar in processor; grind nuts finely. Add flour, powdered sugar, and salt; blend 5 seconds. Add butter and blend, using on/off turns, until coarse meal forms. Add egg yolk and blend until moist clumps form, about 30 seconds. Knead dough into ball. Transfer 1/4 cup dough to small bowl and reserve for another use. Press remaining dough evenly onto bottom and up sides of prepared pan. Pierce bottom all over with fork. Cover pan with foil and freeze crust at least 1 hour and up to 1 day.
Preheat oven to 375°F. Bake crust uncovered until deep golden brown, checking often and pressing sides and bottom with back of fork during first 15 minutes when crust puffs or bubbles, about 20 minutes total. Cool completely.
Place cornstarch and salt in medium bowl. Gradually add 1/4 cup milk, whisking until cornstarch dissolves. Whisk in egg yolks.
Stir sugar and 1/3 cup water in heavy large saucepan over medium-high heat until sugar dissolves. Bring to boil, brushing down pan sides with wet pastry brush. Boil without stirring until syrup is deep amber, swirling pan occasionally, about 10 minutes. Add 2 cups milk (mixture will bubble). Whisk until caramel bits dissolve. Slowly whisk hot milk mixture into yolk mixture; return to same pan. Whisk until pudding thickens and boils, about 2 minutes. Remove from heat. Whisk in butter and vanilla. Transfer to crust. Chill uncovered until cold and just firm, at least 3 hours. Do ahead Can be made 1 day ahead.
Stir 2/3 cup sugar and 1/3 cup water in heavy medium skillet (do not use nonstick) over medium-high heat until sugar dissolves. Bring to boil, brushing down pan sides with wet pastry brush. Boil without stirring until syrup is medium amber color, about 7 minutes. Add almonds and stir to coat evenly with caramel. Immediately scrape coated almonds onto sheet of foil. Working quickly with 2 forks, separate almonds and spread out in single layer. Cool completely. Do ahead Let almonds stand at room temperature up to 6 hours or cover and chill up to 1 day. Break almonds into small clusters before using.
Press bottom of pan up, releasing tart. Whisk cream, powdered sugar, and vanilla in bowl until peaks form; spread over tart. Sprinkle almond clusters with fleur de sel. Garnish tart with clusters.