Mix together melted butter, both sugars, and oil in a large bowl. Add the eggs and vanilla, and whisk very well until combined.
Add the flour (sifted), cocoa, and salt. Mix just until combined. Gently stir in chocolate chips. Pour in a 9x13 pan (metal, not glass is preferred for brownies) sprayed with cooking spray and lined with parchment paper--making sure the ends fold over the edge of the pan.
Bake in a preheated 350 degree oven for 12 minutes. Sprinkle the marshmallows across the top of the brownies evenly.
Bake for 6-8 more minutes, themn remove from the oven to cool. **If you use a different brownie recipe or box mix, follow the baking directions, and add the marshmallows the last 8 minutes of baking.
While the brownies are cooking, make your ganache. In a microwave-safe bowl or glass measuring cup, melt the chocolate chips, butter, and peanut butter for 30 seconds, then in 10-second intervals until you can stir smooth. Pour evenly on top of the marshmallows.
Let sit for about 30 minutes before cutting...if you can wait that long! Remove the parchment paper by holding on to the edges, then cut into pieces.