Preheat oven to 180C
Mix filling ingredients (butter, cinnamon, brown sugar, grated apples with juice squeezed out)
Spread filling over the puff pastry
Roll up the pastry and press down gently
Cut into triangles
Lay triangles on parchment paper on a baking sheet
Mix egg yolk with milk and brush over triangles
Bake for 22-25 minutes until golden and puffy
Dip in cinnamon sugar and serve
