With a sharp knife, roughly chop up the rest of the chocolate (dark and white) into little pieces and set aside. It's ok for them to mix as you'll just be throwing them into the brownie batter later.

Now remember the other dark chocolate in the bowl? Heat it for 90 seconds, stirring after 60 (1 minute).

Add the butter to the semi melted chocolate, make sure the butter is chopped up into a few cubes for easier melting.

Microwave for 30-60 seconds, checking and stirring after 30. At the end of this time, if your chocolate is still in big lumps (it does depend on the strength of your microwave), give it another 30-60 seconds. If they are only tiny lumps, just stir with a wooden spoon until it's all melted and you have a shiny brown mix. Now leave it aside to cool as you get on with the rest.

Mix the milk and the buttermilk together in a jug or large bowl.

With a tabletop mixer, or hand held mixer, add the sugar and vanilla to the melted chocolate and butter mix and beat for about 30 seconds to mix.

Then beat in the flour and milk mixes into the melted chocolate in 3 batches, on the lowest setting. So flour in, beat for 20 seconds, milk, flour, milk and so on. Do this until it's all mixed in. If it gets too thick for your beaters, change to a wooden spoon.

If you don't have a cake mixer, do the above with a wooden spoon (it will get a bit tiring!).

You will end up with a very thick batter. Now's the time to stir in the chopped up chocolates and the nuts if you're using them.

Transfer the brownie batter into the lined tin and spread out with the back of the spoon until it's all fairly even.

Bake for 45 minutes. If your oven tends to be on the hot side, check after 40.