Pour the white wine, white wine vinegar, minced shallot, black pepper, salt, fresh tarragon, and fresh chervil into a small saucepan and simmer until the liquid reduces by half, leaving about 3 tablespoons; then let the mixture cool completely before adding it to the blender with the egg yolks. The reduction is ready when it lightly coats the back of a spoon, and cooling it fully keeps the yolks from tightening too soon.

Melt the butter in a small saucepan or in the microwave, then remove the center cap from the blender lid, run the blender on HIGH, and slowly stream the very hot butter into the yolk mixture in one steady pour. As the emulsion forms, the sauce should turn pale and glossy, and the blender’s sound usually deepens slightly as it thickens.

Keep blending for 30 seconds, then check the texture and blend on HIGH for another 5 seconds only if needed; serve warm right away, or set the blender container in warm water until serving time and finish with the chopped fresh tarragon. The finished sauce should look like a thin mayonnaise, smooth and spoonable rather than runny or greasy.