Preheat the oven to 400°F and line a baking sheet with parchment paper.
Lay the cold puff pastry on the parchment, brush it with melted butter, and sprinkle with flaky sea salt.
Cut it into about 1¼-inch squares. Leave the squares touching.
Bake 20 to 24 minutes, until puffed and deep golden.
Cool 1 to 2 minutes, then break into individual croutons.