Heat your oven to 160°C/140°C fan/Gas mark 3
Grease your baking tin with butter and line the base with greaseproof paper.
Sift the flours and salt together.
Add half of the beaten eggs and half of the flour mixture to the stage 1 ingredients and fold in with a wooden spoon.
Add the rest of the eggs and flour and fold in until fully incorporated, do not over mix but do ensure there are no pockets of flour remaining.
Pour the mixture into your cake tin and gently level out.
Sprinkle the demerara sugar over evenly over the top.
Bake for 1 hour and 20 minute, or until a skewer inserted into the centre comes out clean.
Leave to cool in the tin for 10 minutes before removing and cooling on a cooling rack.