Grease and line tray and set oven to 160 degrees celcius
Melt butter and chocolate on low heat gently for 10mins DON’T STIR
When completely melted, stir gently and leave to cool.
Put eggs, sugar and vanilla into bowl and whisk. Then add the cooled chocolate mixture.
Add flour and cut and fold into mixture (add walnuts if you want to)
Put mixture into tin and smooth out.
Bake for 20-30mins until soft NOT SPRINGY
Cool tin for 30mins then cut and put onto plate or into a container.