Step 1 Preheat oven to 175 °C/ 350 °F (conventional), or 155 °C/ 310 °F (fan assisted) and line a 20 x 20 cm (8 x 8 inch) square pan with parchment paper.
Step 2 Break the chocolate into small pieces and add it and the butter into a small bowl placed over slowly simmering water.
When most of the chocolate has melted, remove the bowl and stir the chocolate.