Preheat oven to 180°C. Line pan (33 x 25 x 5.5 cm) with baking paper.
Melt the butter and chocolate together in the microwave in short bursts on medium.
In a bowl beat the eggs with the sugar and vanilla.
Sift the flour into another bowl and add the salt.
Let the chocolate mixture cool a bit before beating in the egg and sugar mixture, and then the nuts, white choc chips and flour.
Pour into lined tin and bake for 30 minutes (but can often take longer).
When it’s ready the top should be dried to a pale brown but the middle still dark and gooey.