How to Make It
In a food processor, pulse the flour with the confectioners’ sugar and salt. Add the butter and pulse until it’s the size of peas. Add the egg yolk and pulse in 10-second increments until incorporated, about 4 long pulses. Transfer the pastry to a sheet of parchment paper, shape into a disk and cover with another sheet of parchment paper. Roll out the pastry to a 12-inch round. Slide the pastry on the parchment paper onto a baking sheet. Refrigerate the pastry until firm, about 1 hour.
Let the pastry stand at room temperature for 5 minutes to soften. Discard the top sheet of parchment paper and invert the pastry into a 9-inch fluted tart pan with a removable bottom; fit the pastry into the pan and trim the overhang. Prick the pastry all over with a fork and refrigerate for 30 minutes.
Preheat the oven to 400°. Line the tart shell with parchment paper and fill with pie weights or dried beans. Bake for about 20 minutes, until the pastry is set and lightly browned at the edge. Remove the parchment paper and weights and bake the pastry for 5 minutes more, until lightly browned on the bottom. Transfer to a rack to cool completely. Reduce the oven temperature to 325°.
In a microwave-safe small bowl, microwave the chocolate at high power in 30-second bursts, just until melted. Let cool slightly.
Pour the melted chocolate into the baked tart shell, spreading it evenly over the bottom. In a small skillet, stir 1/3 cup of the granulated sugar with the lemon juice and ¼ cup of water over moderately high heat until the sugar dissolves. Cook, without stirring, until the mixture starts to color, about 5 minutes. Continue cooking, stirring constantly with a heatproof spatula, until a lightly golden caramel forms, about 5 minutes. Remove the skillet from the heat and stir in the butter, 1 piece at a time. Stir in the cream and salt, then let the caramel cool to room temperature.
In a medium bowl, whisk the egg yolks with the remaining 1/3 cup of granulated sugar until smooth. Stir the caramel into the egg yolk mixture, then pour the custard evenly over the chocolate in the tart shell. Transfer the tart to a foil-lined baking sheet. Bake for about 30 minutes, until the crust is browned and the filling is still slightly wobbly in the middle. Transfer the tart to a rack and let cool to room temperature. Refrigerate until set and thoroughly chilled, at least 2 hours. Unmold the caramel tart and serve.