Leong likes Valrhona Guanaja's balance of bitter and sweet. It's available at specialty food shops and from valrhona-chocolate.com.

Grease and line a 30x20cm/12x8in rectangular brownie tin. Put the chocolate and butter in a heatproof bowl and put in the oven. Set the oven to 190C/170C Fan/Gas 5 (the chocolate and butter will ...