Put 250g butter or margarine into large mixing bowl. Microwave on high 1 to 1 1/2 minutes until melted.
Stir in 2 cups white sugar and cocoa. Add eggs and vanilla; beat vigorously until well-blended. Stir in flour, nuts and salt. Let batter "rest" for 10 minutes.
Pour into an 20x30cm pan. Place on a plastic trivet or inverted saucer in microwave. Microwave on medium 9 minutes, rotating dish 1/2 turn after 3 minutes.
Microwave on high 3-5 minutes rotating dish 1/2 turn once until top is mostly dry with moist spots and a knife comes out clean. Sprinkle marshmallows evenly over top of cake. Let stand about 5 minutes until marshmallows are slightly melted.
To make icing: Melt 125g butter in large bowl. Stir in 1/3 cup milk, 1/4 cup cocoa and 1/2 teaspoon vanilla. Add 4 cups icing sugar and beat vigorously until smooth. Spread evenly over marshmallows. Let stand on flat surface 30 minutes until slightly warm or cool completely. Makes 16 very rich servings as this is a very dense cake.