Pre-heat the oven to 160°C. Lightly grease a 23cm square cake tin and line with baking paper.
Sift the flour with the baking powder and cocoa. Put the chocolate into a heat proof bowl over a saucepan of hot water and leave until melted (alternatively, microwave on medium power, stirring occasionally, until just melted).
In a large bowl, whisk the eggs with the sugar and vanilla until slightly thickened. Whisk in the chocolate mixture.
Fold in the flour mixture, mayonnaise and finely chopped walnuts.
Pour into the prepared tin and bake in a pre-heated oven at 160°C for about 35-45 minutes.
Leave to cool in the tin before cutting into squares.
These are even more delicious with strawberry coulis poured on top.
Recipe courtesy of Hellmann's