The ultimate makeover: chocolate brownies

Name: The ultimate makeover: chocolate brownies
Prep time: 25 min
Cook time: 30 min
Total time: 55 min
Cuisine: British
Ingredients: 85g dark chocolate , 70% cocoa solids, chopped into small pieces; 85g plain flour; 25g cocoa powder; ¼ tsp bicarbonate of soda; 100g golden caster sugar; 50g light muscovado sugar; ½ tsp coffee granules; 1 tsp vanilla extract; 2 tbsp buttermilk; 1 egg; 100g mayonnaise (I used Hellman's)