Melt butter and chocolate in a small saucepan over a low heat, stirring, until melted and smooth.
Place sugar, eggs and soy sauce in a large bowl. Add melted chocolate and mix until combined.
Sift flour, cocoa and baking powder over the chocolate and stir to combine.
Pour mixture into a 23cm square cake tin lined with baking paper and bake in a moderate oven (180°C/fan forced 160°C) for 30-35 minutes or until set. Cut into serving size squares.
Serve warm or cold with fresh berries, vanilla yoghurt or ice cream if desired.
