A candle shaped and coloured like a block of salted butter, salt crystals included. Unscented.
The joke works because the detail is right: the crystals on the surface, the exact yellow, the way it sits on a dish. Most people pick it up before they realise it is wax.
The Dopamine Lab pours its candles by hand in France, in coconut and rapeseed wax, with a lead-free cotton wick. This one is poured in rapeseed wax alone.
Mostly it sits on a shelf or a butter dish and does its work unlit. It can be lit, on a heatproof plate, but it is made to be looked at.