All-purpose flour, for dusting 1 large egg, at room temperature, separated (see Baking Tip) 2 tablespoons plus 4 teaspoons sugar, divided
2 teaspoons cornstarch ¼ cup water 1 teaspoon freshly grated lemon zest
2 tablespoons lemon juice
⅛ teaspoon vanilla extract
Preheat oven to 375°F. Prepare Crust for Two. Dust a work surface with flour and turn the dough out onto it. Dust the dough with flour and roll into an 8-inch circle, dusting with flour as necessary to prevent sticking. Transfer the crust to a 6-inch tart pan with 1-inch sides and removable bottom (see Sources or Equipment Tip). Press the crust into the bottom and up the sides, folding the overhanging dough back into the pan to form double-thick sides. Prick the bottom and sides with a fork. Bake the crust until lightly browned, 20 to 22 minutes. Set aside to cool for 10 minutes. Increase the oven temperature to 425 degrees . While the crust is cooling, whisk egg yolk and 2 tablespoons sugar in a medium bowl until thick and pale yellow, about 1 minute. Whisk in cornstarch until combined. Bring water, lemon zest and lemon juice to a boil in a small saucepan over medium heat. Remove from the heat. Whisk about half the lemon mixture into the yolk mixture in a slow, steady stream. Whisk the combined mixture back into the remaining lemon mixture in the pan. Return the pan to the stove and cook over low heat, whisking constantly, until thickened and puddinglike, 1 to 3 minutes. Pour into the prepared crust and spread evenly to the edges. Beat egg white in a small bowl with an electric mixer at medium-high speed until soft peaks form. Beat in the remaining 4 teaspoons sugar in a slow, steady stream until stiff peaks form. Beat in vanilla. Mound the meringue over the warm filling; spread to the crust's edges, forming peaks with a rubber spatula. Bake the tart until the meringue is lightly browned, about 8 minutes. Let cool on the baking sheet for 10 minutes, then transfer to a wire rack and cool to room temperature before serving.