Put the flours in a mixing bowl or a food processor. Add the butter and rub in with your fingertips or process until the mixture resembles fine breadcrumbs. Stir in the sugar and chocolate then squeeze the crumbs together with your hands to form a ball.
Press into an ungreased 18 cm (7 inch) shallow square cake tin and prick the top with a fork. Mix the cinnamon and sugar together and sprinkle half over the top. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 20–25 minutes until pale golden.
Remove from the oven and mark into 12 bars. Sprinkle with the remaining cinnamon sugar mix and leave to cool in the tin. Cut the shortbread right through and lift out of the tin. Store in an airtight tin for up to 5 days.