Spray a 9 X 9 square pan with non-stick spray and dust with flour.
Then set pan aside.
Place 2/3 cup oil into a microwavable measuring cup and then microwave for 1 minute.
When warm, whisk in 2/3 cup cocoa powder until completely combined, then set measuring cup aside for 4-5 minutes until cooled.
In a small bowl, measure and add 1/2 cup flour, 1/4 teaspoon baking powder and 1/2 teaspoon salt, and stir to combine, then set aside.
Place 3 eggs, 2/3 cup brown sugar and 1 1/3 cup powdered sugar in the bowl of a KitchenAid mixer.
Then beat on medium speed until fluffy, 2-3 minutes.
Add 1 teaspoon vanilla and then mix on low until just combined.
Add 1/2 of the oil and cocoa mixture and start the mixer on low until just combined, then turn the speed to medium-high for 5 minutes.
Add all of the dry ingredients from the small bowl and then mix on low until just combined.
Add the rest of the cocoa mixture and then mix on medium-high for 1 minute, scraping down the sides of the KitchenAid bowl as needed.
Empty brownie batter into the prepared pan, then using a spatula spread the mixture evenly through the pan.
Bake for 24-30 minutes.
Once done, remove the S'mores brownies from the oven and top them with chocolate and mini marshmallows.
Set the oven to BROIL and then place the S'mores brownies into the oven with the rack in the middle position until the marshmallows are browned.
Remove the S'mores Brownies from the oven and then let the pan cool for 10 minutes before serving!
Notes
Brownies, if you want them Fudgy, are one dessert you do NOT want to overbake.At 24 minutes, check the doneness of the brownies and bake longer if needed.Baked brownies should not look gooey, but slightly firm in the center.