Store-bought puff pastry keeps the prep time for this tart to a minimum. Inspired by turtle candies, the filling is rich, nutty, and chewy.
Preheat oven to 425 degrees. Unfold dough on lightly floured parchment. Roll out into a 1/8-inch-thick rectangle, about 10 by 15 inches. Transfer dough on parchment to a baking sheet. Fold edges in to form a 1-inch border. Pierce bottom of shell with a fork. Brush border with egg wash; sprinkle with sanding sugar. Freeze until firm, about 15 minutes.
Bake shell until puffed and golden brown, 20 to 25 minutes. Press down on bottom with an offset spatula to flatten (leave border puffy). Let cool completely on a wire rack.
Combine granulated sugar and water in a small saucepan; cook over medium-high heat, stirring occasionally, until sugar is dissolved. Continue to cook, without stirring, until syrup comes to a boil, occasionally washing down sides of pan with a wet pastry brush to prevent crystals from forming. Let boil, swirling pan occasionally, until caramel is dark amber, about 10 minutes. Remove from heat.
Gradually add cream (caramel will steam and spatter). Add butter and chocolate; stir until melted and smooth. Pour filling into tart shell and spread to cover bottom. Sprinkle with pecans and salt. Let cool slightly, then refrigerate until set, about 1 hour. Let stand 10 minutes; serve with whipped cream. Tart can be made ahead and stored in refrigerator up to 1 day.