Step by step recipe:

1. 10 min.
   Peel and chop finely 1 shallot.
   
   Chop tarragon and chervil finely.
   
   Reserve one teaspoon of chopped herbs for garnishing.
2. 1 min.
   Put shallot and herbs in a small pan, salt and pepper generously.
3. 15 min.
   Pour in 100 ml white (spirit) vinegar and 100 ml dry white wine.
   
   Reduce over low heat until you are left with about 1/5 of the original volume.
4. 3 min.
   Pour the contents of the pan into a chinois or a strainer, and squeeze herbs to extract the juices.
5. 
   You should get only 2 to 4 tablespoons of this tasty liquid which is the base for your sauce.
   
   This base could be made several days beforehand, and kept in a sealed jar in the fridge.
6. 5 min.
   On the day, melt 250 g butter, remove the foam which rises to the top and discard it.
   
   Your goal is to clarify the butter, removing as far as possbile the white impurities (whey) which will be left on the bottom.
   
   Leave to cool.
7. 5 min.
   Prepare a bain-marie.
   
   Put 2 egg yolks, sauce base, and 1 pinch flour in the bowl over the bain-marie.
8. 10 min.
   Put over low heat, and beat to form a sabayon.
9. 
   Continue to beat until it thickens, and "holds" the whisk.
10. 
    Then add the melted butter, a little at a time, while beating continously.
11. 15 min.
    Your Béarnaise will develop its texture. Continue until all the butter is added, apart from the very last bit in the container with all the impurities (discard).
12. 2 min.
    And finally add the spoonful of herbs.
    
    Serve without too much delay. If your dish is not ready, leave the sauce to wait in the bain-marie, not too hot, with a lid.