Step 1In the bowl of an electric mixer with a paddle attachment, combine the orange zest, butter and sugar. Beat until smooth.
Step 2Add the eggs one at a time, beating after each addition.
Step 3Sift together the salt, baking powder and flour. Slowly add the flour mixture to the butter mixture, beating just until combined. Divide the dough in half and wrap separately. Chill for a few hours or overnight.
Step 1Peel and core the apples. Cut them into one-fourth- to one-half-inch cubes and set aside; you will need 8 cups.
Step 2Heat a large heavy saucepan over high heat. Melt half a cup of sugar at a time, continuing until you have added and melted all the sugar. Cook until the sugar turns an amber color, about 10 to 12 minutes.
Step 3Reduce the heat to low. Add the vanilla bean and seeds and the butter. Stir carefully until combined.
Step 4Add the apples, stirring to combine them with the caramel. The caramel may harden; continue to cook and stir on low heat until the sugar melts, the apples are soft and caramel-colored and the apples have absorbed the caramel sauce. This will take about 30 minutes.
Step 5Pour the caramel-apple mixture into a rimmed baking pan and cool. It can be made several hours ahead.
Step 1Whip the creme fraiche, sugar and zest with the whip attachment of an electric mixer or whisk until thickened. Chill until ready to use.
Step 1Heat the oven to 325 degrees. Place the oven rack in the middle of the oven. Butter a 9-inch springform cake pan and set aside.
Step 2On a lightly floured work surface, roll out the first half of the pastry to a one-fourth-inch thick 12-inch round. Place the pastry into the pan, fitting it into the corners and up the sides.
Step 3Spoon the cooled apple mixture into the pastry in an even layer.
Step 4Roll out the remaining pastry to a one-fourth-inch thick (10-inch) round. Place it on top of the apple filling, pushing the air out the sides and pinching the top and bottom pastry edges together to seal. Save the remaining dough for another use.
Step 5Brush the top with the egg yolk. Score with the tines of a fork in a crisscross pattern to produce a plaid design.
Step 6Bake until the pastry is firm and a golden brown color, about 45 to 50 minutes. Serve warm with a dollop of whipped creme fraiche.