Put the chocolate, butter and corn syrup in a small metal bowl and place this over a small pot of barely simmering water

Stir with a spatula until melted.

Set aside to cool to 90°F to 95°F (32°C to 35°C). This temperature range allows the glaze to be fluid enough to pour smoothly over a cake. For a mousse cake it ensures it will properly set without melting the mousse.