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Heat oven to 180C/fan 160C/gas 4. Pour enough water into a small pan to one-third fill it. Bring to the boil, then remove the pan from the heat. Put the chopped chocolate into a large bowl that will fit snugly over the pan without touching the water. Sit the pan over the water (still off the heat) and leave the chocolate to melt slowly for a few mins, stirring occasionally until it has melted evenly. Remove the bowl from the pan, then let the chocolate cool slightly.
Meanwhile, lightly oil and base-line a 19cm square cake tin that is 5cm deep. Combine the flour, cocoa and bicarbonate of soda. Using a wooden spoon, stir both the sugars into the cooled chocolate with the coffee, vanilla and buttermilk. Stir in 1 tbsp warm water. Break and beat in the egg, then stir in the mayonnaise just until smooth and glossy. Sift over the flour and cocoa mix, then gently fold in with a spatula without overmixing.
Pour the mixture into the tin, then gently and evenly spread it into the corners. Bake for 30 mins. When a skewer is inserted into the middle, it should come out with just a few moist crumbs sticking to it. If cooked too long, the mix will dry out; not long enough and it can sink. Leave in the tin until completely cold, then loosen the sides with a round-bladed knife. Turn out onto a board, peel off the lining paper and cut into 12 squares.