To make cake, put butter in large mixing bowl. Microwave on high 1 to 1½ minutes until melted. Stir in sugar and cocoa powder. Add eggs and vanilla; beat vigorously by hand until well-blended. Stir in salt, flour, and walnuts. Let batter rest 10 minutes.
Pour into an 11¾ x7 ½ inch (or close to it) microwave-safe baking dish (I use glass). Place on a plastic trivet or inverted saucer in microwave oven. Microwave on medium 9 minutes, rotating dish ½ turn after 3 minutes unless your microwave has a turntable. Microwave on high 3 to 5 minutes (5 to 7 minutes at high altitude), rotating dish ½ turn once until top is mostly dry with a few moist spots and pick inserted near center comes out clean. Be careful not to over-bake or cake will become hard around the edges.
Sprinkle marshmallows evenly over top of cake. Let stand about 5 minutes until marshmallows are slightly melted (I put a piece of foil loosely on top.)
Meanwhile: To make frosting, put butter in large bowl. Microwave on high 30 to 60 seconds until melted. Stir in milk, cocoa powder, and vanilla. Add confectioner’s sugar; beat vigorously until smooth. Spread evenly over marshmallows (cake will be warm). Let stand on flat heatproof surface 30 minutes until slightly warm, or cool completely and serve at room temperature. Store any leftovers in a cool place.
Per serving (16 servings): 452 calories, 4 g pro, 66g car, 21 g fat, 117 mg chol with butter, 53 mg chol with margarine, 263 mg sodium.