Blitz the flour and butter in a food processor to the bread crumb stage. Add the sugar,rosemary and egg yolks and blitz again briefly to form a dough. Empty dough out onto a floured work surface and gently fold in the walnuts. Shape into a sausage approximately 4-5 cm in diameter, wrap in cling film and chill for 1 hour or so. While dough is chilling heat your oven to 170 C. Remove from fridge and unwrap and slice into 1/2 cm thick and place on a pre pared tray covered in baking paper. Sprinkle sea salt on top . Bake for 17 minutes or until edges start to turn golden. These biscuits don't spread much , if at all, so you can bake quite a few at a time. Once out leave to cool for 5 minutes on the baking sheet before gently removing to a rack to cool completely. These biscuits work well with your mildest cheddar or the strongest blues.