Put the flours, salt, butter and 50ml/2fl oz cold water in a bowl and gently mix to form a dough.
Transfer the dough to a lightly floured work surface and knead for 5 minutes until smooth. Shape the dough into a square, wrap in cling film and chill for an hour (or freeze for 20-30 minutes).
For the butter layer, cream the butter and flours together using an electric mixer. Sandwich the mixture between two sheets of cling film and roll out to a rectangle the same width as the square of dough, but twice as long. Chill in the fridge for 25 minutes.
Unwrap the chilled dough and butter layer. Place the chilled butter layer, short end facing towards you on a lightly floured surface and place the square of dough in the centre of the butter sheet. Make sure it is positioned neatly and covers almost to the edges. Lift the exposed butter sheet at the top and fold it down over the dough, then fold the exposed butter sheet at the bottom up over the top, so the dough is completely enclosed in the butter sheet.
Place the dough on a lightly floured surface, short end towards you. Roll out to a rectangle, keeping the edges as even as possible. Fold the top quarter down and the bottom quarter up so they meet neatly in the centre. Then fold the dough in half along the centre line. This is called a book turn. Wrap the dough in cling film and chill for 25 minutes.
Remove the dough from the fridge and make another book turn. Wrap the dough in cling film and chill for 25 minutes.
For the filling, mix the granulated sugar and the cinnamon together in a bowl. On a lightly floured surface roll out the pastry to a rectangle as before and sprinkle over the sugar.
Make another book turn to incorporate the sugar, then roll out the pastry 1cm/½in thick, to a rectangle 12x20cm/4½x8in. Roll up the pastry from the short end like a Swiss roll. Wrap in cling film and chill for 25 minutes.
Preheat the oven to 200C/180C Fan/Gas 6. Line 2 baking trays lines with silicon sheets or non-stick baking paper.
Trim the ends of the roll and cut into 8 x 1cm/½in thick slices. Dust the work surface heavily with icing sugar and roll each piece of dough out very thinly, turning to coat in the sugar and to prevent sticking. Place the biscuits on the prepared trays and bake for 5 minutes. Remove from the oven, carefully turn the biscuits and cook for a further 3-4 minutes, or until golden-brown. Transfer to a wire rack to cool.