For the tarragon butter, scald the tarragon leaves with boiling water and leave to infuse for a few minutes. Then drain and dry well with kitchen paper.
Chop with a knife and grind with 20 g butter in a mortar. Add another 80 g butter and 1 pinch of salt and mix everything very well.
Form into a stick in aluminum foil, let it set in the refrigerator and cut into slices.