Butter an 8″ square pan and pre-heat your oven to 180C.
In a saucepan set over low heat, melt butter and chocolate. Let it cool.
Combine sugar and eggs. Add the cooled chocolate mixture.
Add in the flour, cocoa powder and baking powder. Fold in the walnuts
Transfer the batter to the baking dish. Top with peanut butter and using a skewer, make swirls.