01 - Preheat oven to 350°F. Generously grease a 9-inch tart tin with removable base using butter or cooking spray.
02 - In a mixing bowl, rub together flour, cold butter cubes, sugar, and salt with your fingertips until mixture resembles coarse breadcrumbs. Add egg yolk and 1 tablespoon ice water, mixing just until dough begins to clump. Add remaining water only if needed to bind. Briefly knead, form into a disc, wrap in plastic, and refrigerate for 20 minutes.
03 - On a lightly floured surface, roll chilled dough to 11-inch circle. Carefully drape over tart pan, gently pressing dough into corners and up sides. Trim excess dough hanging over edges. Prick bottom all over with fork. Refrigerate for 10 minutes.
04 - Line pastry shell with parchment paper, filling with pie weights or dried beans. Bake for 15 minutes. Remove parchment and weights, then return to oven for 5 additional minutes until bottom is lightly golden. Set aside to cool while preparing filling.
05 - In a medium saucepan over medium heat, combine sliced apples, sugar, lemon juice, and lemon zest. Cook, stirring occasionally, for 10–12 minutes until apples break down into soft, pulpy consistency. Remove from heat. Mash apples slightly with fork, leaving some texture. Stir in butter until melted. Let cool 10 minutes, then vigorously whisk in egg yolks one at a time. Spread mixture evenly into cooled pastry shell.
06 - Using an electric mixer with clean, dry bowl, beat egg whites on medium speed until soft peaks form. Increase speed to high and gradually add sugar, 1 tablespoon at a time, followed by cream of tartar. Continue beating until stiff, glossy peaks form and sugar has completely dissolved. Fold in vanilla extract.
07 - Spoon meringue over apple filling, starting at edges and working toward center to completely seal filling. Use back of spoon or piping bag to create decorative swirls and peaks.
08 - Bake at 350°F for 20–25 minutes until meringue peaks are golden brown and crisp to the touch. Cool completely in tart pan on wire rack. Meringue may crack slightly as it cools—this is normal. Serve warm or at room temperature.