My inspiration came from Bakeaholic, but the recipe can be traced back another couple of steps by following links on each site. As I didn't follow the recipe quantities exactly, and plan to make more changes next time, I'm going to write out the whole recipe.
The biggest problem was the huge difference in stiffness between the chocolate batter and the plain half. The blondie batter was quite runny, but the chocolate batter was almost like cookie dough and very difficult to spread - it will need loosening up next time!
150g sunflower oil
450g light muscovado sugar
330g plain flour
2 teaspoons baking powder
1/2 teaspoon salt
3 large eggs
100g plain chocolate chips
50g chopped hazelnuts
50g 74% plain chocolate, melted.
Preheat the oven to 170C and line a 12 x 8" (30 x 20cm) baking tin with baking parchment. The tin needs to be at least 1.5"+ (4cm) deep.
Mix the oil and sugar until smooth, making sure there are no lumps in the sugar.
Beat in the eggs, one at a time.
Sift together the flour, baking powder and salt and stir into the batter.