How to Make It
In a stand mixer fitted with the paddle, cream the butter with the sugar and salt at medium speed until pale and fluffy, about 2 minutes. Scrape down the side of the bowl. Beat in the flour at low speed until the mixture resembles wet sand, then beat in the egg yolk until the dough just comes together. Scrape the dough out onto a work surface, gather up any crumbs and pat the dough into a disk. Wrap in plastic and refrigerate until well chilled, at least 1 hour.
In a medium saucepan, combine the sugar, oranges and water and bring just to a simmer, stirring to dissolve the sugar. Cook over moderately low heat, stirring occasionally, until the orange slices are very tender and suspended in a slightly thick syrup, about 1 hour and 15 minutes; let cool completely.
On a lightly floured work surface, roll out the dough to a 12-inch round. Without stretching it, fit the dough into a 10-inch fluted tart pan with a removable bottom. Trim the overhang and use it to patch any holes. Refrigerate the tart shell for 30 minutes.
Preheat the oven to 350°. Bake the tart shell for 35 minutes, until golden. Let cool.
In a small saucepan, combine the cream, milk and zest and bring just to a simmer. Remove from the heat and let stand for 30 minutes. Return the mixture just to a simmer, then strain it through a fine sieve into a heatproof bowl. Add the chocolate; let stand for 2 minutes, then whisk until the chocolate is melted and the mixture is smooth. Whisk in the egg yolks, then whisk in the butter and salt.
Put the tart shell on a rimmed baking sheet. Pour the filling into the shell and bake for about 15 minutes, until the filling at the edge is just set and the center is nearly set. Let stand at room temperature for 2 hours.
Drain the candied oranges well, then arrange them on the tart. Cut the tart into wedges and serve.