Preheat your oven to 350 degrees (Fahrenheit), then grease your 9x13 pan with non-stick spray, lard, or shortening. Then set aside.
In your stand mixer (with your paddle attachment) or large mixing bowl (with your wooden spoon), add and cream together your avocado oil, sugar, eggs, and Vanilla extract.
Now, in your other large mixing bowl (with your sifter), sift together your flour, cocoa powder, baking powder, and salt. Stir the flour mixture until completely mixed.
Add your flour mixture to your egg mixture until completely combined. Add the first half of your flour to the egg batter, mix completely, then add the last half and mix completely.
With your spatula, thoroughly scrape the sides. This is when you will add your chopped nuts (if desired). Stir completely.
Pour the brownie batter into your greased 9x13 pan, and with your spatula, completely scrape your bowl clean. Bake at 350 for 30 minutes, or until a toothpick inserted into the middle comes out clean.
When completely baked, pull out of the oven and place onto a cooling grid. Allow to cool in the pan for about 20 minutes. Enjoy serving with ice cream or fresh whipped cream!