Preheat oven to 160 degrees celcius
Grease a tin and line with baking paper. My tin is 7 inches by 11 inches, but anything similar should do!
Melt the chocolate in a bowl over a pan of boiling water. Make sure the water doesn't touch the bowl. Whilst the chocolate is melting, you can do the next 3 steps. Or just microwave the chocolate!
Add the flour, cocoa powder and baking powder together and sieve.
Chop the walnuts, as fine as you want them.
Add the eggs, sugar and vanilla extract together and whisk.
When the chocolate has melted, add to the egg mixture...
...and whisk some more.
Fold the flour mixture into the egg & chocolate mixture.
Add the mayonnaise and fold in.
Add the walnuts (forgot to take a photo!) then pour into the tin.
Into the preheated oven, for 35-45 mins (I keep it to 35 mins so the brownies stay moist in the middle!)
Half way there! Kitchen should be smelling good by now!
Once time is up, take out of the oven to cool and use the baking paper to take out of the tin.
Cut into bite-sized pieces, et voila! Either start eating immediately or chill in the fridge! Or do both!