Preheat the oven to 180oC/350oF/Gas
4. Then line a 33 x 23cm baking tin with greaseproof paper and grease
the paper.
Melt the chocolate and butter over a
very low heat, stirring. Set aside to cool.
Meanwhile, beat the eggs until light
and fluffy. Gradually beat in the sugar. Sift over the flour, salt and
baking powder and fold to combine.
Stir in the cooled chocolate mixture.
Add the vanilla and walnuts.
Measure and set aside 475ml of the
chocolate mixture. For the plain mixture, cream the butter and cream
with an electric mixer. Add the sugar and continue beating until
blended.
Beat in the eggs, flour and vanilla.
Spread the unmeasured chocolate
mixture in the tin. Pour over the plain mixture. Drop spoonfuls of the
reserved chocolate mixture on top.
With a palette knife, swirl the
mixtures to marble. Do not blend completely. Bake until just set, 35-40
minutes.
Turn out when cool and cut into
squares before serving.