This simple shortbread makes a lovely addition to a cookie buffet.
Put butter, sugar, and zest in the bowl of an electric mixer. Mix on low speed 15 seconds. Add flour and salt; mix until dough comes together, 3 to 5 minutes. Mix in cranberries. Divide dough in half; wrap each half in plastic. Refrigerate 30 minutes (or overnight).
Preheat oven to 300 degrees. Transfer dough to a lightly floured surface. Roll out to 1/4 inch thick. Cut out diamond shapes with a 3 1/2-by-2-inch cookie cutter. Repeat with remaining scraps of dough. Space 1 inch apart on parchment-lined baking sheets.
Bake until edges turn golden, 25 to 30 minutes. Let cool on sheets on wire racks.