Pre-heat oven at 180 degrees.
Optional; chop walnuts for up to 5 seconds on Speed 5.
Put butter and broken chocolate into TMX bowl, chop for 5 seconds on Speed 7. Melt for 3 minutes on 50 degrees on Speed 3. Cool slightly.
Add cocoa, vanilla extract and eggs and mix for 30 seconds on Speed 5
Add sugar and flour and mix for 10 seconds on Speed 6
Add chopped walnuts and mix for 5 seconds on Reverse at Speed 4 OR Add UNchopped walnuts, mix for 5 seconds on Speed 4
Pour into lined pan and bake for 30 minutes.
Cool in pan before removing and cutting into squares.