Use the same method as for shellfish essence, but add 1 pound of room temperature butter for the last 30 minutes of processing the shells in the mixer. Cook the buttered shell liquid 5 minutes, strain, and refrigerate the liquid. When it is cold, lift off the butter from the top and use it to enrich sauces, or season it and put it on grilled fish. Use the liquid for shellfish essence.

This butter freezes well and will hold for a month.

If using shrimp shells, grind the shells in a food processor just enough to break them up. If using lobster shells, just put them in the bowl of an electric mixer fitted with the dough hook or paddle, and turn on the mixer at the lowest speed possible for 15 minutes. Wrap some aluminum foil around the top of the bowl and the mixer arm to keep the lobster shells from jumping out of the bowl as they are being crushed. After 15 minutes, add 1 cup of the stock, turn the speed up one notch, and mix for another 10 minutes.

Add the remaining fish stock and mix on low speed until the shells are completely broken up in small pieces and the stock in the bowl has turned red, about another 15 minutes. The long mixing time is necessary to break up the shells gradually and extract the full flavor and color.

Scrape out all the shells and any essence sticking to the sides of the bowl into a sieve placed over a bowl.

Strain completely, pressing down on the shells, reserving the liquid and discarding the shells.

Refrigerate for up to 2 days or freeze for up to a month.