Preheat the oven to 180c conventional. Grease a slice tin, and line the base with baking paper.
Melt the butter in a saucepan over a medium heat. Remove from the heat, and whisk in the sugar. Add the eggs and vanilla essence, and whisk to combine. Sift the flour, baking powder and cocoa into the mixture, add the white and dark chocolate chunks, and nuts (if using), and very gently fold with a wooden spoon or spatula until just combined. Pour the mixture into the prepared tin.
Bake in the oven for about 15-20 minutes then test with a skewer. These brownies are done if the skewer still has a little of the mixture clinging to it. Remove from the oven and cool in the tin before slicing. Store in an airtight container for a week.
