One of the classic French sauces, perfect for beef en croute. It's a bit finicky, but don't let the sauce see your fear. With just a little bit of attention, it'll come together just fine.
2 tbsp white wine vinegar
30g shallot, finely chopped
3 tbsp snipped tarragon leaves
10 cracked white peppercorns
4 egg yolks
250 butter, freshly clarified & cooled to tepid (for instructions on making clarified butter, see the larger recipe for beef wellington/beef en croute)
salt and freshly ground pepper
2 tbsp snipped chervil leaves
In a small heavy-based saucepan, combine the white wine vinegar, shallots, and 2/3 of the tarragon, and the cracked pepper. Bring to a simmer over a low heat and reduce the liquor by half. Leave to cool completely.
Add the egg yolks and 3 tbsp cold water to the cold reduction. Set the pan over a very low heat and whisk continuously. The sauce should emulsify and the temperature increase gradually. Whisk for 8-10 minutes until the mixture has become rich and creamy.
Off the heat, whisk in the cooled clarified butter, a little at a time. When it is all incorporated, season the sauce with salt and pepper, pass it through a chinois (a fine mesh sieve or cheesecloth) and stir in the rest of the tarragon and chervil. Serve without delay.