Make cookie dough: Place butter, flour, sugar, orange zest and salt in the bowl of your food processor fitted with the blade attachment. Process in bursts until crumbly. Tip out on a clean work surface. Add the cranberries and lightly knead a few times JUST until the cranberries are incorporated and the dough forms a ball - do not overwork or you'll get tough shortbread.
Shape dough: Divide the dough in two and shape each half into a log of about 2 1/2 inches diameter. Wrap in plastic wrap and chill in the fridge for at least 30 minutes, and up to overnight.
Bake: When ready to bake, preheat the oven to 350°F and line a few baking sheets. Unwrap the logs of cookie dough and roll in light brown sugar. Slice into 1/2 inch thick slices and place 2 inches apart on the lined baking sheets. Bake in batches for 10-12 minutes, or until lightly browned and crisp around the edges (cookie will still feel soft).
Cool: Cool on the baking sheet for 5 minutes, then remove to a cooling rack and cool completely. Store in an airtight container on the counter for up to a week.