Preheat the oven to 180 degrees/gas 4. In a small saucepan, half-fill it with boiling water and place on the hob under medium heat. Place the unsalted butter and the chocolate (broken into small pieces) into a bowl and put this on top of the saucepan to allow it to melt. Continue to stir with a table knife. Once melted, take the bowl off of the saucepan to let it cool down.