How to Make It
Puree the raspberries in a food processor or blender. Strain the puree through a fine sieve set over a medium bowl and discard the solids. In a small heavy saucepan, bring the sugar and water to a boil. Cook over moderately high heat, without stirring, until a medium-amber caramel forms, about 6 minutes. Reduce the heat to low and carefully stir in the raspberry puree until smooth. Stir in the butter, then remove from the heat and stir in the salt. Let the raspberry sauce cool completely.
In a food processor, pulse the flour with the sugar and salt. Cut the butter into 8 pieces and add to the flour; pulse until the mixture resembles coarse meal. In a small bowl, whisk the cream with the egg yolk, lemon juice and zest. Add to the flour mixture and process just until large clumps of dough form. Pat the dough into a disk, wrap in plastic and chill until firm, about 30 minutes.
On a lightly floured surface, roll the dough 1/8 inch thick and fit in a 10-inch fluted tart pan with a removable bottom. Freeze 15 minutes, or until firm.
Preheat the oven to 350°. Line the dough with foil and fill with pie weights or dried beans. Bake the tart shell for 25 minutes, then remove the foil and weights and bake for 5 minutes longer, or until the pastry is lightly browned on the bottom. Cover the shell loosely with foil if the sides begin to brown too quickly. Let cool completely on a rack.
Arrange the raspberries in concentric circles over the bottom of the tart shell. In a small bowl, whisk the cream with the egg yolks, sugar and scraped vanilla seeds. Pour the custard into the tart shell. Bake in the middle of the oven for 50 to 55 minutes, or until the custard is set. Let the tart cool on a wire rack for at least 30 minutes. Serve with fresh raspberries and the raspberry-caramel sauce.