Preheat the oven to 350 degrees F. Grease an 8-inch square baking pan with gluten-free cooking spray.
For the flour blend: Whisk together the rice flour, tapioca flour, potato starch, xanthan gum and salt in a large bowl. Reserve the remaining flour blend for another use.
For the brownies: Whisk together the sugar, baking powder, salt and 1/2 cup of the gluten-free flour blend in in a small bowl. In a medium microwavable bowl, combine the chocolate chips, oil, cocoa powder, cinnamon, cayenne and 3 tablespoons water. Microwave on medium power until the chips are almost melted, about 30 seconds. Whisk until smooth. Let cool to room temperature, about 3 minutes.
Whisk the eggs into the chocolate mixture until the batter is smooth and shiny. Whisk in the flour mixture just until blended. Spread the batter evenly in the prepared pan. Bake until set, about 30 minutes. Let cool completely on a rack.