Cream the butters and sugar together. Then add the vanilla, and slowly add the flour and kosher salt. Shape the dough into a log and refrigerate for at least half an hour. Preheat oven to 350F. Cut slices of the log approximately 1/4 inch thick and put cookies onto parchment-lined baking sheets. Bake for 15 to 20 minutes until the edges are browned. After removing from the oven sprinkle each of the cookies with a touch of fleur de sel.