Step 1If using a mixer, cut the butter into 1-inch cubes. Blend with the sugar, salt and flour at low speed until the mixture resembles coarse meal. Add the cream and mix until the dough comes together.
Step 2If mixing by hand, combine the sugar, salt and flour and chill in the freezer, 30 minutes. Cut the butter into 1-inch cubes. Add to the chilled ingredients and toss until coated with flour. Pinch the flour and butter together until the butter is in thin flakes. Rub small amounts of the mixture between your fingers until it resembles coarse meal. Add the cream and turn it in with a rubber spatula, turning the flour over repeatedly. When absorbed, press the dough against the bottom of the bowl.
Step 3On a lightly floured surface, roll the dough into a 13-inch circle. Fold the dough into quarters and lift into an 11- or 12-inch tart pan. Unfold and settle it into the pan. Gently press the dough against the sides, being careful not to press against the bottom. Trim off the extra dough. Chill until firm, about 20 minutes.
Step 4Heat the oven to 400 degrees. Line the shell with heavy foil, covering the edges and bottom. Poke holes with a fork over the bottom, piercing both foil and dough. Bake until the inside of the shell no longer looks raw, 20 minutes. Remove the foil and bake until the shell is golden brown and has pulled away from the sides, 3 to 5 minutes more.
Step 1Heat the oven to 350 degrees.
Step 2Stir together the sugar and water in a 3-quart heavy-bottomed saucepan. Bring to a boil over medium heat and cook without stirring until half the sugar has dissolved and the syrup is a light mahogany color (350 degrees on a candy thermometer), about 30 minutes. Swirl the pan during cooking if the syrup is not darkening evenly. Remove from the heat and very carefully pour in the cream (the hot syrup may splatter). When the bubbling stops, stir and set aside to cool at least 15 minutes. (This is important because if the syrup is too hot when you add it to the eggs, it will cook them and ruin your filling.)
Step 3Toast the walnuts on a baking sheet in the oven, 5 minutes. Chop them into 1/2-inch pieces.
Step 4Beat the eggs at medium-high speed, 1 minute. Mix in the caramel. Stir in the walnut pieces. Spoon the mixture into the baked tart shell with a slotted spoon. Pour any remaining caramel over the top and smooth the surface with the back of the spoon. Bake 30 to 35 minutes. The filling will feel soft when it comes out of the oven but will set as it cools.