Put rack in center of oven and heat to 180ºC Generously grease 9x13 inch pan. Coat with flour, tapping out the excess. Use mixer to cream sugar and butter until fluffy. Add eggs and mix until thick and lemon colored, about 2 minutes. Mix in chocolate and vanilla. Add cocoa, flour and salt. Slowly mix in batter until combined. Stir in chips and walnuts. Lay marshmallows sideways in prepared pan, allowing 4 cm border around the inside edge of the pan and spacing them evenly. Spoon batter (it's very thick) over marshmallows to hide them as much as possible. Evenly fill in center of pan with remaining batter, not adding batter to space between marshmallows and sides of pan (batter will spread during baking and reach sides). Bake until toothpick inserted into center comes out moist from melted chocolate (not batter), 25 to 35 minutes. Let cool completely on rack. Cut into dozen squares. Refrigerate until chilled. Use wide metal spatula to carefully transfer brownies to serving platter. Wrap airtight. Can be made a day in advance and refrigerated or frozen up to 3 months. Serve chilled or at room temperature. 1:59 PM