55g Butter 225g Sugar 2 Large Eggs - beaten Juice and finely grated zest of 2 lemons (or 4 limes)
When it's cold, store in the fridge - it will last about 1 month.
Put a bowl over a pan of simmering water, and add the butter and sugar until dissolved. Add the lemon/lime juice and zest, and then the eggs whisking as you add them to prevent curdling. Let it cook gently stirring until it is thick and like custard (be patient it takes about 10 - 15 minutes!). Take off the heat and put into a suitable jar.