• 1/4 cup white wine vinegar
• 1/2 cup dry white wine
• 3 sprigs tarragon, leaves finely minced, stems reserved separately
• 3 sprigs chervil, leaves finely minced, stems reserved separately (optional; if not using add an extra sprig of tarragon)
• 1 small shallot, roughly chopped
• 1/2 teaspoon whole black peppercorns
• 2 egg yolks
• Kosher salt
• 1 1/2 sticks (12 tablespoons) unsalted butter
Combine vinegar, wine, herb stems, shallots, and black peppercorns in a small saucepan. Bring to a simmer over medium-high heat and lower heat to maintain a gently simmer. Cook until reduced to about 1 1/2 tablespoons of liquid, about 15 minutes. Carefully strain liquid through a fine mesh strainer into a small bowl, pressing on the solids with the back of a spoon to extract as much liquid as possible.
Combine vinegar reduction, egg yolk, and a pinch of salt in the bottom of a cup that barely fits the head of an immersion blender. Melt butter in a small saucepan over high heat, swirling constantly, until foaming subsides. Transfer butter to a 1-cup liquid measuring cup.
Place head of immersion blender into the bottom of the cup and turn it on. With the blender constantly running, slowly pour hot butter into cup. It should emulsify with the egg yolk and vinegar reduction.
Continue pouring until all butter is added. Sauce should be thick and creamy. If it is thin and runny, transfer to a large bowl set over a pot of barely simmering water.
Whisk constantly and vigorously until sauce is thickened. Season to taste with salt. Whisk in chopped tarragon and chervil.
Serve immediately, or transfer to a small lidded pot and keep in a warm place for up to 1 hour before serving. Bearnaise cannot be cooled and reheated.