Preheat oven to 350˚F. Line a sheet pan with foil and spray with cooking spray. Rub foil with a paper towel to cover surface well. Spray lightly again. Set aside. Place an (approximately) 18-inch piece of parchment paper on a heatproof work surface.

Add butter to a large non-stick pan and melt over medium heat. Add sugar, balsamic vinegar and salt. Continue to cook, stirring continuously until sugar is dissolved. This will take 3-4 minutes. Add walnuts and rosemary, stir gently with a heat-proof spatula to coat walnuts with sugar mixture.

Turn out onto prepared pan and place in oven. Bake for 15 minutes (It may take a bit longer), removing from oven and gently stirring every five minutes until nuts are deep golden brown. Remove from oven and immediately turn onto prepared parchment paper. Working quickly, separate nuts with two forks. Sprinkle generously with flaky sea salt. Allow to cool completely. Store in an airtight container.