Preheat the oven to 170'C.
Line a baking tray with greaseproof paper.
Melt the butter and mix with the brown and white sugars.
Whisk together the eggs and the vanilla and slowly mix into the butter and the sugar mixture. Add both flours, salt, bicarbonate of soda and the vanilla. Mix well and then spoon the mixture onto the baking tray (one large spoon of mixture per cookie) . Leave large gaps between the cookies so they don't stick together.
Bake the cookies in the oven for 14-15 minutes. Allow to cool a little on the tray before serving.
«Halfway through baking, sprinkle the cookies with caster sugar so they form a crunchy crust on top.»