In a stand mixer fitted with a paddle attachment, cream the butter, sugars, vanilla, rosemary and lemon zest. About 2-3 minutes until light and fluffy
Add in your egg yolk and reserve the white for later. Mix just until combined.
lastly, add in the flour and salt and mix util there are no dry pockets left.
Dump the dough onto parchment paper or plastic wrap and shape it into a log about 2-2.5 inches thick. Doesn't have to be perfect. Wrap and refrigerate for 2-3 hours so it can firm up.
Once the dough has firmed, preheat your oven to 350° and brush the whole log with remaining egg white and roll in demerara sugar, until the outside is completely coated. Using a serrated knife, slice the cookies ½ inch thick. Place onto a sheet pan lined with parchment paper and top with salt & lemon zest (optional). Bake for 13-15 minutes until the edges are golden brown. Remove and allow them to cool for at least 20-30 minutes before enjoying.