Step one: melt the butter and chocolate in a bain-marie (basically, a heatproof bowl (so not a plastic one) over a saucepan of just-simmering water – don’t let the bottom of the bowl come into contact with water). Turn the water off when it’s just bubbling and stir the mixture gently until it’s combined. Take it off the heat and allow to cool to room temperature (if you pour very hot chocolate into the eggy mixture, you risk getting blobs of scrambled egg in your brownies. Ick).