Browned butter, or beurre noisette, is the secret to the crust's rich, nutty flavor. Martha made this recipe on "Martha Bakes" episode 601.
Make the tart shell: Melt butter over medium heat. Continue to cook, stirring constantly, until deep golden brown, about 11 minutes. Immediately pour browned butter into a large bowl, and let cool to room temperature, about 10 minutes. Stir in vanilla.
Preheat oven to 350 degrees. Whisk together flour, baking powder, sugar, and salt. Gradually stir into browned butter mixture. Press dough into a 9-inch tart pan with a removeable bottom. Refrigerate until firm, about 15 minutes. Using the tines of a fork, pierce bottom of tart shell all over.
Line tart shell with parchment and fill with pie weights or dried beans. Bake until edges are set and golden, 15 to 18 minutes. Remove weights and parchment and continue baking until bottom of crust is dry and golden, about 5 minutes more. Remove from oven and let cool completely on a wire rack.
Make the lemon curd filling: Place 2 1/2 teaspoons cold water in a small bowl. Sprinkle gelatin on top; let stand until softened, about 5 minutes.
In a medium saucepan, whisk together sugar, lemon zest, and egg yolks; whisk in lemon juice and salt. Add butter and place pan over medium-high heat. Cook, whisking constantly, until butter has melted, mixture is thick enough to coat the back of a spoon, and small bubbles form around edge of pan, about 5 minutes.
Remove pan from heat while continuing to whisk. Add softened gelatin, whisking to combine. Strain curd filling through a fine-mesh sieve into the tart shell, spreading evenly, and refrigerate for at least 1 hour before serving; wash and dry the saucepan.
Make the meringue topping: Just before serving, fill the same saucepan with about 2 inches water and bring to a simmer. Combine sugar, egg whites, and salt in a heatproof standing mixer bowl. Set bowl over (not in) simmering water, and whisk until mixture is warm to the touch and sugar dissolves, 2 to 3 minutes. (Test by rubbing mixture between your fingers; the mixture should be smooth.)
Attach bowl to standing mixer fitted with the whisk attachment. Whisk on high speed until stiff, glossy peaks form and mixture is cool, about 10 minutes. Whisk in vanilla and cornstarch.
Transfer meringue into a large pastry bag fitted with a large round tip; pipe meringue to cover filling, as desired. Place under broiler or use a kitchen torch to brown. Serve immediately.