In a large Bowl, mix together flour, sugar, salt, baking powder, eggs and softened butter until well combined.
Peel, core and roughly chop apples. Combine in a large bowl with brown sugar, lemon juice and ground cinnamon.
Grease pan (26cm diameter) and coat work flour. Take 2/3 of the dough and evenly press into baking pan, pulling up the dough along the edges.
Mix hazelnuts with remaining dough. With your hands, crumble up dough and spread evenly over apples.
Put baking pan into preheated oven (180°C) and bake for 50 minutes. If crumbles start getting too dark, cover cake with aluminum foil for remaining baking time. Enjoy hot with a scoop of vanilla ice cream.