Combine flour, baking powder, sugar, and salt in a large bowl
Cut in the cold butter until the mixture resembles coarse crumbs
Add the egg yolks and mix until a dough forms, adding milk only if needed
Press the dough evenly into a greased and floured 9x13" pan
Pre-bake the dough at 350°F (175°C) for 10–15 minutes until lightly set
Peel, core, and grate apples and strain out extra juice
Mix apples with sugar and cinnamon
Spread apple mixture evenly over the dough
Beat egg whites until soft peaks form
Gradually add sugar until stiff and glossy
Gently fold in potato starch or cornstarch
Spread meringue over the apple layer
Mix flour, sugar, and butter, rubbing with fingers until coarse crumbs form
Sprinkle crumble evenly over the meringue
Bake at 350°F (175°C) for 50–60 minutes until crumble is golden and meringue is set
Cool completely before slicing
