Preheat oven to 350° Prepare 9x9 baking pan by spritzing with cooking spray and then cutting parchment paper to fit the pan plus two inches over the edge on two sides.
The Wet Ingredients
Add vinegar to plant milk and let sit for about five minutes or until it starts to sour. It will begin to coagulate a little. Add in vanilla and set aside.
If using coconut oil, melt and set aside. If using other oil, measure out and set aside.
The Dry Ingredients
Add flour, sugar, cocoa powder, baking powder, salt, and cornstarch to a medium/large sized mixing bowl.
Stir well so that all cocoa powder is broken up. Sift if necessary. I used a fine mesh strainer to sift. (See post above for photos and instructions.)
Mixing Dry and Wet Together Along with Add-Ins
Add soured milk mixture and oil to dry ingredients all at once. Mix by hand until all dry ingredients have been moistened. Do not overmix.
Fold in the chocolate chips and fine-chopped walnuts
Turn out into prepared pan and smooth the top.
Set 9x9 pan on a larger baking sheet and put in oven. Bake for 25 -28 minutes.
You will know the brownies are done when they are slightly firm to the touch in the middle. Do not overcook or they will be hard. They are undercooked if they are jiggly in the middle. Remove from oven.
Sprinkle on the toppings: the chocolate chips, marshmallos, and chopped walnuts. Return to oven for five more minutes.
Allow to cool in the pan for about 15 minutes. Then carefully lift out the entire cooked browie, using the sides on the parchment paper to assist and place on cooling rack.
Continue cooling for at least an hour before cutting. They are messy...
Use a light-flavored oil such as canola oil or (refined) melted coconut oil.
Allow to cool completely before cutting. It's hard to be patient and I rarely am.