Mound flour on a surface and create a well.
Work butter and salt into the flour until grainy.
Incorporate 125ml ice cold water without overworking the marbled dough.
Roll into a 2.5cm thick rectangle and chill for 20 minutes.
Roll pastry into a 40x20cm rectangle on a floured surface.
Fold into thirds, turn 90 degrees, and repeat rolling and folding.
Repeat the folding process to achieve four total turns.
Wrap and refrigerate for at least 30 minutes.
Preheat the oven to 220°C.
Roll pastry to 4mm thickness, cut into three 7.5x15cm rectangles, and dust with icing sugar.
Bake pastry for 15 minutes while preparing the filling.
Sauté apples in butter, then add 45ml water.
Simmer with vanilla, blackberries, and lemon juice until pulpy.
Cool the fruit mixture in the fridge.
Whisk double cream until firm peaks form and transfer to a piping bag.
Slice-cooled pastry sheets in half widthways to create thin rectangles.
Layer fruit mixture and piped cream onto the first pastry sheet.
Add a second pastry layer and repeat the fruit and cream layers.
Top with a glazed pastry piece and slice into portions.