Ingredients  
3 egg yolks  
½ tsp white peppercorns  
250g butter 
3 tbsp white wine vinegar  
3 tbsps water 
1 bay leaf  
½ tsp lemon juice 

Equipment needed 
Thermos flask - I favour one for soups which is wider and shallower as your sauce will be thick and it will be much easier to get out 
A bowl which fits nicely on to a saucepan or a food processor 
A metal whisk 

Method  
First things first if you are not going to be eating this immediately then prepare your flask.  Boil your kettle and fill your flask with the water and put on the lid. 

If you have a food processor you can use this to amalgamate the sauce ingredients.  Otherwise put a smallish saucepan on to the heat and put in some of your boiling water, check that the bottom of the bowl does not touch the water - this is important.  Put on the heat to low/medium  to keep hot.   

Cut your butter into pieces and put into a medium saucepan and melt over a low heat. 

Meanwhile roughly crush your peppercorns, either in a pestle and mortar or with the base of your saucepan.  Put the peppercorns, vinegar, water and the bay leaf into a very small pan and simmer for a few mins to reduce to roughly 2 tbsps of concentrated flavour.  This will only take 3-5 minutes max.  Be careful because once it starts to reduce it will reduce very quickly. 

Either in your food processor or your bowl put your egg yolks and just pulse a few times or whisk gently.   

Once your vinegar mixture has reduced your butter should be melted.  Skim off any froth that might be on the butter and discard.  Turn the heat off of both the vinegar and butter.  Take a small jug and strain the vinegar.   

Now either turn your food processor on or put your egg bowl over the water and start to whisk.  Slowly start to add the vinegar reduction all the time whisking.  Once all of the vinegar is added you can start to add your butter.  By this point you may want to just get it finished but you need to have lots of patience because you need to add the butter really super slowly or the mixture split and you will want to cry.  If your saucepan has a pouring lip adding the butter will be easier but this is not essential.  Very slowly start to pour the butter in (even slower than you are imaging) while continually whisking.   

This can be tricky with 1 person as sometimes the bowl also moves around but is doable.  If you have somebody to hand nab them.

Keep slowly adding the butter, stopping when you get to the very bottom before the milk solids go into your sauce.  Whisk in the lemon juice and then taste to check you have enough salt and adjust if needed.  

Either serve and eat or pour into your flask.