Preheat the oven to 180°C / 350°F / gas 4. Prepare your lined tin – I use a 20.5 x 30.5cm one. Put the butter in a heatproof bowl along with the dark chocolate. Place a small amount of boiling water into a saucepan and rest the heatproof bowl with the butter and chocolate on top, then melt the contents on a low heat. You don’t want the water to boil. Once melted set aside.