Preheat the oven at 180℃/350℉ (if you are using a convection oven its 165℃/330℉)
Melt the chocolate in the microwave(quicker) or au-bain-marie (safer). If you are using the microwave make sure to stir every 20-30 seconds till melted. If you are using a chocolate bar a little tip is to break it up in some smaller chucks to help speed it up. You don't want to burn the chocolate by heating it up to much. Slow and stead wins the race here.
Melt the butter in a different bowl and let cool of a bit.
Add the sugar to the butter and whisk with the electric mixer till its combined.
Add the eggs and the vanilla extract and continue whisking till it becomes lighter of color and is combined well.
Add the melted chocolate. Make sure its cooled off!! If its too warm it shall start cooking the eggs. Mix this well.
Switch over to a spatula. Add the flour and salt together and sift this in to the mixture. Then, you guessed it, mix. Over mixing doesn't happen fast with these. Because of the white chocolate.
Add the chopped chocolate to the mixture and fold in. I personally like chopped cause you get some nice bits of melted chocolate in the mix.
Line the baking tin with baking paper and fill with the mixture. Put the tin in the oven.
The baking time should take about 25 minutes. I say should cause these are a little tricky. They will seem underdone for a long time. If the top is nice and golden brown.
The hardest step!!!! Let it cool off fully before you try to take it out of the tin. Or dig in with a spoon. I advise, after its cooled, to put it in the fridge and let them chill a little. Then use the baking paper to pull it our or just tip it over on your work surface.
Divide in to "equal" pieces (the baker gets first choice!!) and enjoy. Keep in the fridge.