Preheat oven to 350° and butter a 9 x 13 baking dish, set aside.
In a medium saucepan, melt butter and chocolate chunks. Once melted, add cocoa powder and stir. Remove from heat and let cool for 5 minutes.
Next, add sugar and vanilla, stir.
Add eggs, one at a time, into mixture and stir, then add flour and stir just until combined.
Stir chocolate chips and walnuts into mixture and stir.
Pour mixture into prepared baking dish, pop in the oven and bake for 28 minutes, give or take a minute or two.
Remove from oven and let cool a bit. Cut into desired sizes and enjoy with a scoop of vanilla ice cream!