Grease an 8 inch square metal pan and line with foil. Grease bottom only. You could also use parchment, but I've been testing with foil. These are very sticky brownies, so you do need to grease the bottom.
In a large mixing bowl, stir or whisk together the flour, both sugars, cocoa powder and salt. Take your time with this and make sure the dry ingredients are evenly blended.
Make a well in the center and add the eggs, oil, butter, vanilla and water. Whisk together liquid ingredients, then stir everything together, adding in the chocolate chips, to make a thick batter.
Spread the batter in the prepared baking pan and pat down flat. Sprinkle walnuts over the top. Cover with foil and chill 12 hours or overnight.
When ready to bake, preheat the oven to 325 degrees F.
Spread batter in the pan and bake for 35 to 42 minutes or until internal temperature tops out at about 210 degrees F. 38 minutes is how long mine take.
Allow the brownies to cool for several hours, then loosen slightly from edges of pan. If you have time, I recommend giving them a good chill before slicing.