Preheat the oven to broil with the rack near the top.
Clean the mussels, discarding any that are open. Bring the liqueur to a boil in a large pot, and add the mussels, cover, and cook 3 minutes, or until opened. Drain and discard any mussels that have remained closed.
Mash the butter with the chives and tarragon in a bowl. Put a pinch of the herbed butter on each mussel. Sprinkle over the breadcrumbs. Grind over some pepper. Broil until the crumbs are golden, about 2 minutes. Serve the mussels with lemon wedges for squeezing.
Recipe courtesy of Laura Calder