Melt the butter in a non-stick frying pan over a medium heat. Cut out four 40cm squares of greaseproof paper and brush with a little melted butter. Trim the leeks, then julienne or cut into matchsticks and sauté in the butter for 6 to 8 minutes, or until lightly golden. Season well.
Divide half of the leeks between the paper. Place a salmon fillet on each, season, and spoon over 2 tablespoons of vermouth, 1 tablespoon of stock and the remaining leeks. Scatter over the capers. Finely chop and scatter over the dill. Fold up the sides of the paper and scrunch up to make secure parcels.
Place the parcels on a baking sheet and cook for 15 minutes, or until the salmon is cooked. Open the parcels at the table.