1/2 lime (or lemon), juiced
4 tbsp salted butter
NB, this freezes well so you make it into a sausage shape using clingfilm, then just cut off a slice as and when needed.
Soften the butter in a pan, a microwave (or by balancing the bowl on a lid of a boiling pot of water containing something like sweetcorn). Finely chop the chilli and squeeze the lime. Whisk the lime and chilli into the butter and refrigerate to cool. You may find not all of the lime juice incorporates; that is fine.
Use as flavouring on boiled or grilled sweetcorn (you can brush the liquid mixture straight onto the corn, then pop in under the grill to brown for a couple on minutes). It is fantastic when used to pep up plain boiled rice, potatoes, peas, beans…. whatever you have really!