Using a mixer, blend melted butter and cocoa until smooth. Add flour and sugar; beat together. Mix in eggs and vanilla (and nuts if you are using them) and mix until combined (be sure not to over mix).
Pour batter into a greased 9x13" baking pan. Bake for 20-25 minutes or until a toothpick inserted into the middle comes out clean (I am in a high altitude location- I have heard that for lower altitude, you may need to bake for just a little longer).
Let the brownies cool for about 10-20 minutes and then frost while still warm.
Mix all ingredients together and frost as desired.