For the ice cream dissolve the cornstarch in 50ml full-fat milk, leave to rest.
In a saucepan combine the leftover milk, the double cream, sugar, pinch of salt, cook on medium heat until almost boiling, stir in the cornstarch milk and cook constantly stirring until thickened. Remove from heat, cool to room temperature.
For the tarragon take the leaves off the stems, blanch the leaves in boiling water for 5-10sec, drain through a strainer, plunge in ice water, then drain again and squeeze all of the water out.
Combine the ice cream base, the blanched tarragon and extra virgin olive oil in a blender and blend until smooth and AGAP (as green as possible:) Strain the mixture through a very fine sieve, cool in the fridge overnight.
Churn the ice cream in your ice cream maker according to manufacturer's instructions and store in an airtight container in the freezer for at least 4 hours until set or spoon right away.