Crust: In a medium bowl, whisk together flour, Clabber Girl Baking Powder, and salt. Using a pastry blender or your fingers, cut the butter into the flour until the mixture resembles coarse crumbs. Do not over mix - you should see pea size chunks of the butter. Stirring with a fork, add the ice water and vinegar until the dough is moist enough to hold together when pinched between your thumb and forefinger. You may need more or less ice water to moisten the dough and the dough need not form into a ball.
Gather the dough, flatten into a disc and wrap in plastic wrap. Refrigerate for one hour or overnight. When ready to bake, let the pastry dough sit at room temperature for about 15 minutes. On a very lightly floured surface, use a rolling pin to firmly and evenly whack the dough, turning frequently, to create a round disc about 10-inches in diameter. If the edges begin to crack, use one hand to pinch it back together and the other hand to press and flatten the dough. Finish shaping the dough with a firm rolling action, from the center out to the edges of the dough, to about 1/8-inch thickness, pressing together any cracks that may form.
Roll the dough gently over the rolling pin and place over the tart pan, unrolling it into place. Press the dough firmly into the pan, especially in the corners. Trim the dough about 1/2-inch above the edge of the pan, to allow room for shrinkage while baking. Prick holes evenly over the bottom of the pan and place in freezer while preparing the apples.
Preheat the oven to 400 degrees F. Place a rack in the middle of the oven.
Filling: Toss the apples with the lemon juice and set aside. Melt the butter and brown sugar over medium heat, stir in 1/4 c. of the cream and bring the mixture to the boiling point, stirring constantly for 1-1/2 minutes. Remove from heat and whisk in the remaining 2 Tbsp. of cream.
Stir the egg with a tsp of water and brush the mixture over the bottom of the tart pan, place the apples evenly and decoratively over the bottom of the pan, and drizzle the caramel over the apples. Fold the dough over the edge of the apples and brush any exposed surface with the egg wash.
Place the tart pan in the oven, directly on the rack. You may wish to place a cookie sheet with parchment or aluminum foil below it, in case any of the filling bubbles over. Bake for 12 minutes and reduce the temperature to 350 degrees F. without opening the oven door. Bake for another 30 to 40 minutes, until the filling is bubbly in the center and the crust is golden brown.
Remove and cool for about 20 minutes before serving.
Yield: 1 10-inch apple tart.