1. In a saucepan, melt Plugrá Butter and allow to cool partially.
2. In a heavy nonaluminum 1-quart saucepan over low heat, combine vinegar, salt and pepper. Reduce by half to about 2 tablespoons and remove from heat. Cool to room temperature.
3. Whisk the egg yolks into the vinegar reduction. Place the saucepan over low heat and whisk constantly until yolks have turned white and thickened visibly. Remove from heat and immediately begin whisking in the warm butter a drop at a time. As your progress, after the sauce has clearly "taken," you can gradually increase the amount of butter you add until you have incorporated all the butter.
4. Strain the sauce through a warm chinois (run it under hot water just before using) into a warm, clean saucepan.
5. Finalize the sauce by whisking in the lemon juice. Taste and add salt or lemon juice as needed.