To make the croissant dough, put the flours, sugar, salt and yeast into a large bowl and mix together so everything is evenly distributed. Make a well in the middle and pour in 265ml of lukewarm water and the room-temperature butter. Mix together to form a shaggy dough. Using either an electric mixer fitted with a dough hook, or by hand, knead the dough until smooth and elastic – this will take 7-10 minutes using an electric mixer or about 15 minutes by hand. Put the dough in a bowl, cover, and set aside for 45 minutes-1 hour or until doubled in size. Tip out the dough onto a worksurface, press into a flat rectangle and put this onto a baking-paper-lined tray. Cover and freeze for 30-40 minutes or until firm.