Preheat oven to 160°C.
Line the base of a brownie baking tin with baking paper.
Microwave 180g of the chocolate and the butter on high for about 2 minutes (microwaves varyin wattage, so stir and check to prevent burning). Stir and set aside.
Mix flour and cocoa powder together and set aside.
Using an electric mixer on high speed, beat together the sugar, eggs and vanilla until pale and creamy. This should take about 5 minutes.
Pour the cool chocolate and butter mix over the egg~sugar mix and gently fold together. Do not overmix. Use a sieve to dust flour and cocoa mix over chocolate and egg mix and again gently fold. Again do not over mix.
Gently fold in the chocolate chips/chunks and nuts (if using) evenly through the mix.
Pour mix into prepared tin, making sure of even spread of chocolate chunks and bake for about 25-30 minutes. Gently shake the tin and if it has a slight wobble, but not a fluid wobble, the brownie is cooked. If not, bake for an additional 5 minutes.
Remove and place on a cooling rack. Allow to cool completely before removing, slice into squares.
* To make crossed on your brownies, make a batch of royal icing by mixing 1 egg white with 1 1/4 cups pure icing sugar and 1/2 teaspoon lemon juice. Put into a snap lock bag, snip a corner and start piping your crosses. Piping crosses before slicing makes it easier.