Béarnaise sounds tricky, but made in a blender it’s really not. And if it does go wrong? Tip it away and start again. It’s not the end of the world.
The secret is the gastrique, a vinegar reduction that gives béarnaise its distinctive taste. Simmer a roughly chopped banana shallot, half a bay leaf, six chopped tarragon stalks (save the leaves) and 4 tbsp white wine vinegar until reduced to about a tablespoon. Strain, add a drop of water and let it cool.
Clarify 250g unsalted butter. Blend 4 egg yolks with the gastrique, then with the motor running pour in the butter very slowly. That makes about 300ml. Finish with lots of chopped tarragon (and chervil if you have it). Mine even sat happily for 45 minutes and was refreshed with a teaspoon of water.