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.