Preheat the oven to 190°C.
Mix the butter with dill, chives, salt, and pepper in a small bowl.
Shape the butter into a cylinder, wrap in cling film, and freeze.
Cut the chilled butter into four equal pieces.
Wilt the sorrel in 3 tablespoons of water in a saucepan over medium heat, then drain.
Divide the pastry into eight pieces and roll each into 18cm x 12cm rectangles.
Place salmon on four pastry pieces, topping with a butter slice and wilted sorrel.
Brush edges with egg yolk, cover with remaining pastry, seal tightly, and brush tops with egg.
Bake for 25 minutes until the pastry is puffed and salmon is cooked.
Serve the hot pastry parcels with fresh salad leaves.