Preheat oven to 160°C (325ºF). Place the chocolate and butter in a saucepan over low heat and stir until smooth. Allow to cool slightly.
Place the sugar, eggs, cocoa, flour and baking powder in a bowl. Add the chocolate mixture and mix until combined. Pour into a 20cm-square slice tin lined with non-stick baking paper. Bake for 50 minutes or until an inserted skewer withdraws clean. Allow to cool slightly in the tin before slicing. Serve warm or cold. Makes 16 slices.