Preheat oven to 350 degrees F. and line an 8x8 inch baking dish with parchment paper.
Place butter into a large mixing bowl and microwave until melted and hot, about 45 seconds or so. Pour in chocolate chips and stir until mostly melted. It's ok, if it's not perfectly smooth. You might have to put back in microwave for another 30 seconds to help melting. Add flour, salt and sugars. Mix a few times then add eggs and yolk and oil. Mix until well combined then add chocolate chunks.
Transfer batter to prepared baking dish, spreading evenly. Top with mini marshmallows and pretzels. Bake for 35-40 minutes, until cooked through.