Place the eggs, lemon juice and salt into the Thermomix bowl and blend 1 min/speed 8.
Meanwhile, bring the butter to a rolling boil on a medium high heat on your cooktop.
When the butter is boiling, set the timer on the Thermomix for 1 minute. Add the boiling butter onto the lid of the Thermomix bowl (be warned it will spit a little bit as the MC floats to allow the butter through and into the bowl). Blend 1 min/speed 8.
Serve immediately over eggs, asparagus, grilled steak, make eggs benedict, or add the following options.
To make this sauce become a bearnaise, add some tarragon leaves in the same process as adding the butter. Other fresh herbs can make a difference to this sauce as well.
For a foyot (for steak or chicken) try adding instead of salt, 1-2 tsp Chicken Stock Powder (from this site). This in no way is traditional, but gives a deep umami flavour to the sauce that is addictive! Alternatively, you could add a tsp of Umami Paste (also on this site) to the sauce for a similar result.