Preheat oven to 350 degrees F. Line a 9 inch square metal pan with nonstick foil.

Melt the butter in a large saucepan set over medium heat. Reduce heat to low, add chocolate and chocolate chips to partially melted butter; stir until smooth and completely melted. If you prefer, you can melt the butter and both chocolates together in the microwave. Let cool.

Meanwhile, in the bowl of a stand mixer fitted with a whisk attachment (or just using a handheld mixer), begin beating the eggs on medium-low. Gradually add the sugar, then increase mixer speed and continue beating on high until mixture is thick and pale.

Add the melted and cooled chocolate mixture, vanilla and salt. Stir until blended, then add flour and stir with a heavy duty scraper until smooth. Lastly, add the marshmallows and nuts. Make sure your batter is not so warm that it will melt the chips, then stir in the remaining ½ cup of chocolate chips.

Transfer mixture (it will be very thick) to the pan and spread evenly. Bake in a 350 degree oven for 25-30 minutes.

Remove from the oven and let cool completely or if desired, cool quickly in a water bath. To do this, put the 9 inch square pan in a larger pan and fill the larger pan with ice water so that it comes up about ⅔ of the way (or at least halfway) up the sides of the smaller pan.

Allow the brownies to cool completely, then lift from the pan and cut into squares.