The small, lobster-like crustaceans that the French call langoustines are among the best you can find, with a similar but even sweeter flavor and more tender meat than lobster. I find it incredibly cool that their official name in English is Norway lobster. And even though we can perhaps not claim to have invented them, some of the best specimens are to be found off the coast of my native Norway.
If you have live langoustines: Kill the langoustine by placing it on your cutting board backside up, holding it still with your hand, grabbing it behind the head. Place the tip of a sharp knife on top of its head where a cross is naturally formed. In one determined movement drive the knife through its head. This will kill it instantly. Using your hands, remove the head and claws of each langoustine (do not throw them away; freeze and save them for a stock or soup). Carefully remove the shell from each tail, leaving the fin for decoration. Stick single leaves of tarragon to the backside of each tail, creating a stripe pattern.
In a medium frying pan, heat the butter over medium high heat until completely melted and foamy, but not brown. Place the tails in the skillet backside down, and sear for about 2 minutes. While searing, place tarragon on the other side of the tails, as before. Trying not to remove the tarragon leaves, turn and sear for 1 minute, longer if you want to make sure that is thoroughly cooked through. Place on a serving dish.
To make the sauce, add the white wine to the hot frying pan. Using a spatula, scrape out the solids that have formed on the bottom and stir them into the wine, as the wine reduces into about half in volume. Work rapidly so the wine does not evaporate completely. (This process is called deglazing and creates a very good base for any sauce.) Add the cream, mustard and saffron, and simmer for 1-2 minutes, until the sauce has thickened a little. Drizzle the sauce over the langoustine tails, add a couple of fresh tarragon leaves, and serve immediately.