In a mixer with a paddle attachment, whip the butter until very light and fluffy. Add the sugar and continue whipping, then add the rum and vanilla and mix. Toss together the coconut, flour and salt then add to the butter mixture and mix until blended. Butter a small sheet pan (9 by 12-inch), and line it with plastic all the way up the sides. Spread the dough evenly into the pan, cover it with plastic and freeze. Cut frozen dough into 2-inch strips and roll in flour. Wrap and freeze or cut into1/8-inch bars and place on parchment or silpat-lined sheet pan. Bake in a conventional oven at 250 degrees F for about 30 minutes until crisp but barely colored.