Cut woody bottoms off the asparagus and discard. Peel if necessary. Preheat a pot of boiling water for steaming.
Allow butter to come to room temperature. Make this sauce not more than 5-10 minutes before serving. Placeorange juice, Curacao, vinegar, mint and white wine in a small saucepan. Bring to the boil and reduce by 2/3; you should have about 1/3 of a cup of liquid left.
Remove the mint from the reduction and pour into a stainless steel bowl with the egg yolks, whisking as you pour. Whisk the egg yolk mixture over boiling steamer water until the mixture is frothy, thick and creamy: Don't let it go to scrambled eggs.
Keep the whisk going and remove the bowl from heat when the sabayon is ready. Gradually whisk in the soft butter, a bit at a time. Store in a warm (not hot) place, whisk occasionally. This sauce can separate so be careful.