For the pastry, combine first measurements of butter and flour. Roll out and place in the fridge overnight to rest. Dissolve salt and sugar in the water. Place melted butter and second measurement of flour in a mixing bowl, pour in the water mixture, then quickly bring everything together to form a rough dough. Turn out the dough onto a bench lightly dusted with flour and form into a rectangular shape. Place the first butter dough on top in the centre. Fold the dough in half to cover the butter and roll out the dough evenly to form a rectangle. Rotate the dough half a turn. Fold and roll out again to form a rectangle. Repeat the procedure. In total, this will be two times of the half fold. Cover and rest in the fridge for 1 hour. Take the dough out of the fridge, dust the bench lightly with flour, soften the dough with a rolling pin, then roll out, make the book fold and repeat the half turn. Rest in the fridge for another 2 hours. Roll out the pastry into a square. Pastry should be rolled out big enough to go over the baking mould. Place on a baking tray lined with baking paper and place another tray on top to prevent it rising. Bake in the oven at 200°C for 6 minutes and 30 seconds, then turn the oven down to 180°C and bake for another 5 minutes and 20 seconds. Turn the pastry so it bakes evenly and bake for another 8 minutes. Sprinkle icing sugar over the pastry and continue baking until the pastry caramelises on the top.
For the caramel, place sugar in a heavy-based saucepan over medium-high heat and cook, swirling the pan occasionally, until dark brown and caramelised, then carefully add butter and swirl to combine. Add apples and cook until tender. Remove from heat and place apples in a 47cm x 6.5cm x 3cm mould.
Simmer the caramel remaining in the pan to reduce by two-thirds, then add half to the mould and reserve the remainder.
Preheat oven to 180C and bake for 25-30 minutes. If there is any caramel overflow, scoop it up from the tray and add to top. Remove from the oven, cool to room temperature, then refrigerate overnight.
Meanwhile, for the saikyo miso cream, whip cream and miso together until soft peaks form.
Place pastry on top of caramelised apple and flip over. Remove the mould. Cut into 10 portions and serve topped with remaining caramel and saikyo miso cream.