Cut the butter into cubes and let sit at room temperature for about 10 minutes.
Whisk baking powder and salt into the flour to distribute evenly. Rub in the butter cubes.
Add powdered sugar, lemon zest, egg yolks and sour cream. Combine the dough in the bowl.
Transfer the dough to the kitchen surface and give it a few kneads.
Divide the dough in half and form disks. Wrap them in cling film and chill for at least 1 hour.
Grease and flour the cake pan. Roll out one half of the dough.
Blind bake this layer for 10 minutes at 180 °C [350 °F]. Set aside to cool.
For the filling, peel and grate the apples, and then weigh them. Add in sugar and simmer on low heat until apples become mushy.
Allow the apples to slightly cool and then add walnuts, rum and cinnamon.
Beat the egg whites to soft peaks and gently fold them into the filling. Spread the filling on the bottom layer of the pie.
Roll out the other half of the dough and cover the filling.
Bake for 30 minutes at 180 °C [350 °F]. Let the pie cool slightly before cutting it.
Dust with powdered sugar and enjoy!
