Cream the butter and sugar until smooth. Add the flour and salt and work the mixture with your fingers or a pastry blender until all the flour is incorporated. Wrap the dough in plastic wrap and chill in the refrigerator for 2-3 hours.
When you're ready to bake, preheat the oven to 325 degrees F. On a lightly floured surface, roll half the dough until it's 1/4-inch thick. Using a small knife, cut out rectangular or slightly rounded tombstone shapes, about 3 inches tall. Place on an ungreased baking sheet. Roll and cut the second half of the dough, and place the baking sheets in the refrigerator for 10 minutes. You can gather together scraps and re-roll them.
Bake for 15-20 minutes, until the shortbread is very lightly golden. Do not let the cookies brown. Remove to a wire rack and cool.
When the cookies are completely cool, melt the chocolate chips in the microwave in a heatproof bowl for 1 minutes. The chips may look unmelted, but stir them with a spoon to see if they have actually softened before putting them in for another minute, if necessary.
Spoon the melted chocolate into a zipper-lock bag. Seal the bag, pressing out any air. Use a toothpick to make a very tiny hole in one corner of the bag to release a thin stream of chocolate for writing. Decorate tombstones with crosses and RIP or other writing as desired. Allow the chocolate to set before stacking the cookies.