Perfect béarnaise

1 Put the shallot and 1⁄2 the tarragon in a small pan with the vinegar and set over a high heat. Bring to the boil, then reduce the heat and simmer until the vinegar has reduced to around 1 tbsp. Strain through a sieve, discarding the shallot and tarragon. Set aside the vinegar.
2 Melt the butter in a small pan. Put the egg yolks in a food processor and whizz with the strained vinegar. With the motor still running, very slowly pour the hot butter into the processor so it emulsifies with the egg and becomes smooth and thick. Pour into a serving jug or bowl; stir in the remaining chopped tarragon and season. Delicious served with fish or steak.