Melt the butter in a microwave-safe bowl (or on the stove-top) until melted and bubbling.TIP: Make sure the bowl is covered to avoid splatters.
Place the egg yolks, lemon juice, Dijon mustard, cayenne pepper (optional), salt and pepper into a blender (or Thermomix). Mix on HIGH speed for 5 seconds until combined. Scrape down the sides and repeat for a further 5 seconds. If using a Thermomix: Mix for 10 seconds, Speed 6. Scrape the sides then repeat.
Reduce the blender to medium speed and very slowly let the butter pour into the blender until combined and smooth. If using a Thermomix: Reduce to Speed 3 and very slowly pour the hot butter through the MC hole and continue mixing until combined and smooth.
Serve immediately.
Notes
RECIPE NOTES & TIPS
The butter needs to be VERY HOT AND BUBBLING when it's added to the egg yolk mixture, otherwise it won't cook the egg yolks and the sauce won't thicken.
When melting the butter - make sure the bowl is covered to avoid splatters.
Cayenne pepper adds a little heat if you like that, but it is optional.
Pour the butter in SLOWLY - otherwise the sauce can split.
Use room temperature egg yolks - this makes blending with the hot butter easier, and the sauce less likely to split.
This sauce is best served immediately after making - either warm, or at room temperature.