Preheat the oven to 350 degrees F. Line an 8x8-inch baking pan with parchment paper or grease generously with butter.
In a large mixing bowl whisk the butter, both sugars, egg, salt, and cinnamon together until smooth
Add the flour, oats, white chocolate chips and cranberries. Stir gently just until combined - the mixture will be thick.
Spoon the batter into the pan and spread out evenly from edge to edge.
Bake 25 minutes until lightly golden around the edges - a toothpick from the center should come out mostly clean with a few wet crumbs - not wet batter - attached.
Cool completely.