Mix flours, salt, butter, and 50ml cold water to form a dough.
Knead dough for 5 minutes, shape into a square, and chill for 1 hour.
Cream butter and flours together, roll into a rectangle, and chill for 25 minutes.
Place the dough square inside the butter layer and fold to enclose completely.
Roll into a rectangle, perform a book turn, and chill for 25 minutes.
Perform a second book turn and chill for another 25 minutes.
Roll out the pastry and sprinkle with mixed cinnamon and granulated sugar.
Perform a book turn, roll to 1cm thickness, and roll up like a Swiss roll.
Preheat oven to 200°C and line two baking trays.
Cut into 1cm slices, roll thinly in icing sugar, and bake for 9 minutes total.