In a bowl, mix flour, baking powder, sugar, and salt. Add butter and rub until crumbly.
Add egg and sour cream, then knead into a smooth dough. Divide into two portions and chill for 30 minutes.
For the filling, cook grated apples with sugar, cinnamon, and lemon juice for 5–10 minutes until slightly softened. Let cool, then stir in breadcrumbs.
Roll out one portion of dough and place it into a greased baking pan.
Spread the apple filling evenly over the dough.
Roll out the second portion and place it on top. Prick the surface with a fork.
Bake in a preheated oven at 180°C (350°F) for 35–40 minutes until golden brown.
Let cool, dust with powdered sugar, and cut into squares.
