This is a good base recipe, however I feel that it needs tweaks.
The brownies went down a storm at work, despite the fact that I was a tad disappointed by the texture and consistency of them. When I have made brownies or blondies in the past, they have not been quite so indulgent and more sticky cake-esque, than moist butteriness.
I had reduced the butter to 200g, which I felt was the right amount, however should I make this again, I will change the flour to self-raising, increase the flour to 180g, adding 90g to the dark chocolate rather than 50g to even out the quantities of dark to white, and also provide more absorbency for the butter.
I found making the checkerboard effect rather tedious and messy, and found that the two different batters didn't bind together very well (perhaps because I didn't swirl them enough for the marbling), so I would probably exchange 100-120g of dark chocolate for 80-100g of milk chocolate, which may also reduce the richness of them. I was not able to get golden caster sugar here in Sweden, so mixed 150g light brown soft muscovado sugar, with 150g of white caster sugar. I may increase the white to muscovado ratio, as the latter also contributes to the richness of the brownie. To lighten the load of the arrangement of the batters, I will probably layer the batters, and then attempt some vigorous swirling for the marble effect.
The baking time and temperature definitely needs to be reassessed; after reading others' comments on the site, I checked the temperature and baking time on other successful brownie recipes that I have used and set the temperature at 180 deg C (non-fan assisted), and baked for 45 minutes. From the moistness of the brownie, it wouldn't have hurt it to be in the oven for another 5 minutes, however this temperature and time has been successful in the past. When I used a fan oven, I baked brownies at 170 deg C for 40 minutes, plus another 5 with the temperature off, but the fan on. I always cool brownies in their tin for 15 minutes before turning them out.
With this recipe I buttered and lined my tin (with light buttering on top of the paper), however I found that with the lack of 'stability' or binding-ness, the mixture stuck to the paper, and being someone who hates wasting things, I felt too much came away with the parchment.
I will probably try it again, however will have to experiment with it a couple of times before I think it is worthy of achieving an 8/10!