Preparation Cook lobster in large saucepan of boiling salted water until just opaque in center, about 7 minutes. Using tongs, transfer lobster to work surface and cool slightly (reserve water in pan). Remove lobster meat from shell. Cut meat into 1 1/2-inch pieces. Bring 1/3 cup milk to simmer in medium saucepan. Add lobster meat and tarragon; cover and set aside. Return reserved water in pan to boil. Add potatoes and boil until potatoes are tender, about 20 minutes. Drain potatoes; return to pan and mash. Add butter and lobster mixture to mashed potatoes and stir until butter melts. Season potatoes to taste with salt and pepper. (Can be made 3 hours ahead. Cover and refrigerate. Before serving, rewarm over low heat, stirring often and thinning with more milk, if desired. Or transfer to microwave-safe bowl, cover with plastic, and rewarm in microwave until heated through, about 2 minutes.)
Preparation Cook lobster in large saucepan of boiling salted water until just opaque in center, about 7 minutes. Using tongs, transfer lobster to work surface and cool slightly (reserve water in pan). Remove lobster meat from shell. Cut meat into 1 1/2-inch pieces. Bring 1/3 cup milk to simmer in medium saucepan. Add lobster meat and tarragon; cover and set aside. Return reserved water in pan to boil. Add potatoes and boil until potatoes are tender, about 20 minutes. Drain potatoes; return to pan and mash. Add butter and lobster mixture to mashed potatoes and stir until butter melts. Season potatoes to taste with salt and pepper. (Can be made 3 hours ahead. Cover and refrigerate. Before serving, rewarm over low heat, stirring often and thinning with more milk, if desired. Or transfer to microwave-safe bowl, cover with plastic, and rewarm in microwave until heated through, about 2 minutes.)