Preheat oven to 350 degrees F and line an 8 inch square metal pan with foil or parchment paper.Melt the butter in a large (3 quart) heavy saucepan set over medium heat. Reduce heat to low, add chocolate and stir until smooth. If you are using a metal saucepan and butter is very hot, you can remove the saucepan from the heat and then add the chocolate, letting it melt over residual heat.
Remove from heat when melted.With a large rubber spatula or wooden spoon, stir both sugars and salt into the chocolate mixture. Stir in the eggs one at a time.
Add the flour and stir until well mixed.Fold in the chocolate chips, marshmallows and pecans.
Pour the batter into the prepared pan and spread it evenly.
Bake for 28 to 30 minutes, or until a skewer inserted in the center comes out moist but clean.
Let cool in the pan on a cooling rack for about 30 minutes, then transfer to the refrigerator and chill for a few hours.When brownies are very firm and cold, lift from the pan and cut into squares.
