Line a 23cm/9in baking tray (with shallow lipped edges) with non-stick baking paper or use a disposable foil tray of the same size.
In a microwave-safe bowl, heat the chocolate pieces on medium in 30-second bursts, until fully melted, stirring after each burst. Alternatively, melt the chocolate in a heatproof bowl set over a pan of gently simmering water (ensur