Blanch tarragon in generous amounts of boiling water for 30 seconds, shock in ice water, drain.
Bring the milk to the boil, add together with tarragon in blender and blend on full speed for 1 minute until smooth.
Add trehalose and mix again briefly.
Pass through a fine sieve.
Melt white chocolate and emulsify tarragon cream onto chocolate in 3 additions.
Transfer to a jug and blend again, then blend in mascarpone and vanilla.
Finally blend in cold cream & mould on top of madeleine sponge.