Cream butter and sugar together in the bowl of your stand mixer. (You will need a stand mixer for this unless you are super strong).
Add eggs, cocoa, vanilla, baking powder and salt. Mix until well combined.
Add xanthan gum, all flours and starches and mix until combined. Add milk and mix until integrated.
Last, once the batter is all together (it will be VERY sticky and somewhat stiff), add nuts and choc chips. Stir on low until integrated.
Grease a 13x9 glass baking dish. Scoop batter out with a silicone spatula (you'll be thankful you did) and do the best you can to work it into the corners and make it sort of level. The levener will do the rest if you get it close.
Bake at 350 F for 45-75 minutes. At 45, pull and check for doneness by inserting a toothpick into the center. If it pulls out clean, it's completely done (I like mine a little raw, so I pull at 45 minutes). Allow to rest for at least 20 minutes before cutting.