1 cup sugar
1 cup heavy whipping cream
SWEET PASTRY DOUGH
Put flour on smooth work surface, add sugar and mix to combine. Add butter and toss to coat. Crumble butter with fingertips to coarse cornmeal-like consistency, keeping butter well-coated with flour to prevent it from becoming greasy.
Form mixture into mound and make large well in center. Add egg yolks and cream to well. Briefly stir eggs and cream together with fingers, then begin to draw in butter-flour mixture. When completely incorporated, dough will be very sticky. Gather dough into loose ball. It may be necessary to add as much as 2 tablespoons extra cream to make dough moist enough to shape.
Lightly dust work surface and 1 hand with flour. Using heel of floured hand, begin to smear small portions of dough away from you. (Process is messy and sticky, but it blends ingredients together with least working of dough.) When all dough is smeared out, use dough scraper to collect dough and reincorporate it into one mass. Scrape work surface clean.
Clean and completely dry hands. Lightly dust hands and work surface again, knead dough few times, form into smooth ball, flatten slightly into disk and wrap in plastic wrap. Refrigerate until firm, 2 to 4 hours, before rolling to desired shape. (Dough can be kept in refrigerator up to 2 days or frozen several weeks.) Roll out dough to fit 2 (10-inch) tart pans. Refrigerate 1 hour. Line pastry shells with parchment paper, then weight with dry beans. Bake at 350 degrees until edges are lightly browned, 25 to 30 minutes. Remove beans and parchment paper and let cool.
Cut 2 circles of parchment paper 10 inches in diameter, fold each into eighths, cut into 16 individual wedges and set out on smooth work surface. Combine sugar and egg whites in large mixing bowl over saucepan of simmering water and whisk until smooth and warm to touch, 1 to 2 minutes. Off heat, push flour through fine-mesh strainer into egg white mixture and whisk to combine completely. Beat at high speed 3 minutes. Let stand 10 minutes. Spread meringue in 1/8-thick layer over parchment wedges, taking care to completely cover parchment.
Distribute wedges on baking sheets about 1 inch apart and bake at 350 degrees until meringue rises and top is smooth, shiny and lightly brown, about 10 minutes.
Peel paper away from meringue; paper should separate cleanly. Let wedges cool.
CHAMPAGNE VINEGAR SAUCE
Combine sugar and water in large saucepan over medium heat and bring mixture to boil without stirring. Scrape pulp and seeds from vanilla bean into saucepan.
When mixture boils, it will throw sugar onto sides of pan. At this point, wash down sides of pan with pastry brush dipped in water. When sugar starts to color, after 4 to 5 minutes, swirl pan gently if needed to ensure even coloring. Continue to cook mixture until it turns translucent caramel color, just before it begins to smoke. If mixture becomes opaque and slightly grainy, continue to cook, gently swirling pan until sugar completely melts and mixture becomes clear. If mixture seizes completely and becomes solid and white, it must be discarded.
Remove from heat and add Champagne vinegar; mixture will spatter, and part of it may seize and harden.
Return pan to medium-high heat and cook until sugar melts again and mixture reduces slightly, 3 to 4 minutes. Remove from heat and stir in butter. Mixture will foam up and become cloudy. Return to heat and cook 3 to 4 minutes.
Using fine-mesh strainer, strain sauce into mixing bowl and place bowl in larger mixing bowl of ice water to cool sauce. Discard vanilla bean. Sauce must be whisked occasionally to prevent butter from separating and hardening. As soon as it is slightly thickened, about 5 minutes, remove sauce from ice bath. Sauce should have appearance and texture of thin, emulsified caramel sauce. Serve sauce at room temperature.
Whisk together lemon juice and sugar in medium saucepan over medium heat and cook until sugar dissolves, 1 to 2 minutes.
Whisk eggs and cream in large stainless-steel mixing bowl just to combine.
Slowly add lemon juice-sugar mixture to egg-cream mixture and whisk to incorporate completely. Strain custard through fine-mesh strainer into second large mixing bowl.
Divide custard between pre-baked Sweet Tart Pastry shells and bake on middle rack of oven at 275 degrees until center still jiggles slightly when gently shaken, 30 to 40 minutes. Let cool 15 to 20 minutes.
Cut tarts in wedges same size as meringue pieces, top with meringue and serve each portion with few tablespoons Champagne Vinegar Sauce.
Makes 2 (10-inch) tarts, 16 servings. Each serving: 438 calories; 123 mg sodium; 126 mg cholesterol; 15 grams fat; 73 grams carbohydrates; 6 grams protein; 0.07 gram fiber.