The melt-in-your-mouth succulence of a butter-poached lobster tail is pure bliss. The term "butter-poaching" is a bit misleading—it doesn't mean you're poaching in pure butter. Butter solids separate from butterfat well before the poaching temperature of 180 to 190 degrees Fahrenheit, so you must use a water-supported emulsion. Adding water keeps the butter creamy throughout the cooking process.
Making a Clean Break
Butter-poaching lobster is a specialized technique—you wouldn't perform the same method on chicken breasts, for example. First, you can't simply submerge lobster tails in melted butter, poach them until cooked through, and expect superb results. You must flash-blanch them first by pouring a solution of boiling water and acid over the tails to quickly coagulate the surface of the meat without cooking the interior. Without flash-blanching, the fat between the shell and meat congeals and fuses them together during poaching; if that happens, you won't be able to get the tail meat out of its shell in one glorious piece.
Place the lobster tails in a food-grade container; pack them in closely without compressing them. Barely cover the lobster with cold water, then pour that water off into a bowl or measuring cup; the amount of water it takes to cover the tails is the amount you need to flash-blanch them.
Measure that same amount of water and pour it into a saucepan, along with 1/2 cup white vinegar or lemon juice per 4 cups water. This acid cuts the richness of the lobster fat at its surface and frames its delicate taste.
Bring the water-acid mixture to a boil, then pour it over the lobster tails. For every 1 pound of lobster, steep it for a timed 1 1/2 minutes.
Using tongs, pull the tails from the water, and lay them flat on a sheet pan lined with paper towels. Let them cool to room temperature. Do not hold the tails under cold water after removing them from the acid-water solution, or they may become overcooked.
Separating the Meat
Using a cotton kitchen towel to shield your hand from the hot shell, gently press the tail flat with your palm. Twist the tail to the right or left to make an opening to press the meat through with your fingers. Insert two fingers into the tail of the lobster to release the tail meat. Set the meat aside while you prepare the beurre monté.
Making Beurre Monté
The technique of beurre monté—to mount, or add small amounts, of butter in an immiscible liquid—defines butter-poached lobster. The technique is fundamental: Heat water to a gentle simmer and add butter, about 1 teaspoon at a time, while whisking to incorporate. Control the heat—if you let the emulsion boil, it breaks.
For every 2 pounds of lobster tails, bring 2 tablespoons purified water to a simmer and cut 1 pound butter (also for every 2 pounds of lobster tails) into teaspoon-sized pieces. With a whisk in one hand, drop a pat of butter in and slowly whisk; let it melt gently as the boiling subsides. Continue mounting the rest of the butter into the water 1 pat at a time.
Add the spices and aromatics of your choice as you mount the butter. But don't go heavy with the secondary flavorings—it's best to keep the flavors simple and true to their subject. Minced shallot and a squeeze of fresh lemon juice provide all a quality lobster tail needs.
The beurre monté should reach 180 to 190 degrees Fahrenheit. (Check temperature with an instant-read thermometer.) Once it does, add the tail meat to the beurre monté. If you intend to use the tail shells as serving vessels, cook them separately in boiling water until they turn red.
Poach the lobster just until it reaches 140 degrees Fahrenheit; it will continue to cook to its minimum internal temperature of 145 degrees Fahrenheit.
Remove the lobster from the beurre monté and serve as is. Or, for a hands-on experience, cut the shell in half lengthwise and place the cooked tail meat in the bottom half of the shell. Spoon a tablespoon or so of beurre monté over the presentation.