Cut the salmon into pieces and mash together with the egg and heavy cream for about 3 minutes until a firm consistency. Season with salt and pepper and add the chives or dill.
Divide the salmon cream among 4 well-buttered ramekins and smooth the surface. Bake in a bain-marie at 180 °C for about 15 to 20 minutes.
In the meantime, prepare the hollandaise sauce. Boil the vinegar with the peppercorns, bay leaf spice and mace until 2 teaspoons of liquid remain. Strain into a large enough bowl and whisk in the egg yolks and a small knob of butter. Place the bowl in a water bath and slowly add the remaining butter over a gentle heat. Do not allow the sauce to get too hot.
To serve, turn the mousse out onto small hot plates, add a little hollandaise sauce and garnish everything together with a sprig of dill.