Prepare the tart dough by combining flour, powdered sugar, and salt in a food processor. Pulse to mix, then add cold butter cubes and pulse until mixture resembles coarse crumbs. Add egg yolk and pulse until dough starts to come together, adding ice water only if necessary.
Form the dough into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes or up to 2 days.
Preheat your oven to 375°F (190°C). Roll out the chilled dough on a lightly floured surface to about ⅛-inch thickness and carefully transfer to a 9-inch tart pan with removable bottom. Press dough into corners and trim excess.
Blind bake the tart shell by lining it with parchment paper and filling with pie weights or dried beans. Bake for 15 minutes, then remove weights and parchment and bake for another 5-8 minutes until light golden. Reduce oven temperature to 325°F (165°C).
Make the caramel base by combining sugar and water in a medium saucepan over medium heat. Without stirring, allow mixture to cook until it turns amber (about 8-10 minutes). Watch carefully as caramel can burn quickly.
Remove caramel from heat and slowly whisk in the heavy cream (be careful, it will bubble vigorously). Add vanilla bean seeds or extract and return to low heat, stirring until smooth.
Whisk egg yolks in a large bowl, then gradually add the warm caramel cream mixture, whisking constantly to prevent the eggs from curdling.
Strain the custard through a fine-mesh sieve into a clean bowl to ensure silky smoothness.
Place the par-baked tart shell on a baking sheet and carefully pour in the custard filling, leaving about ¼ inch from the top.
Bake the tart at 325°F (165°C) for 25-30 minutes, or until the custard is set around the edges but still has a slight jiggle in the center. The internal temperature should reach 170-175°F (77-79°C).
Cool the tart completely on a wire rack, then refrigerate for at least 1 hour or up to overnight.
Just before serving, sprinkle the top evenly with granulated sugar and caramelize with a kitchen torch until the sugar melts and turns golden brown. Let the sugar harden for 1-2 minutes.
Carefully remove the tart from the pan, slice with a sharp knife dipped in hot water, and serve immediately for the best texture contrast.