Place all the dry ingredients together in a large bowl and gently mix them with a whisk. Use a small knife to cut the butter over the dry ingredients. Cut into small irregular pieces no larger than 2cm in length.
Gently rub the ingredients between both hands, until the mixture looks like coarse breadcrumbs. Add the egg yolk and work the dough as little as possible until all the ingredients are mixed together into a smooth dough that no longer sticks to the sides of the bowl.
Cut a piece of parchment paper about 20cm x 40cm and empty the ball of dough onto it. Roll it out into a cylinder of a 3cm diameter. Wrap with the paper and place in the freezer for 1 hour, until the dough is very hard.
Preheat the oven to 175°C/350°F. Unwrap the dough and cut it into disks ½ cm thick. Line two baking sheets with parchment paper and arrange the cookies in rows leaving 3cm between each one. Start checking on them 10 mins after you have put them in the oven and bake until they are golden brown. Serve with a dip or simply as a snack. Enjoy!