In a microwave bowl, melt butter; stir in cocoa.
With a mixer or whisk, beat eggs until light.
Add sugar, then chocolate mixture, and blend.
Add flour, vanilla, and nuts. Mix well. Batter will be thick.
Pour batter into greased (or wax-lined) 8 or 9 inch square pan.
Bake at 350 degrees for 30 to 35 minutes.
Cool before cutting into squares.