Preheat the oven to 350 degrees. Spray a 13- by 9-inch baking pan with cooking spray and set pan aside.

Combine the sugar, butter, eggs, and vanilla in a large mixing bowl and stir vigorously with a rubber spatula until well combined. Add the cocoa powder, baking powder, and salt and stir vigorously until smooth. Scrape down the sides and bottom of the bowl and stir again briefly,

Fold in the flour until just incorporated, then stir in the nuts.

Scrape the batter into the prepared pan, smooth the top, and bake 27 to 32 minutes, until a toothpick inserted into the center of the cake comes out with just a few crumbs.

Remove cake from the oven and sprinkle the marshmallows over the top. Return to the oven for 2 to 2 1/2 minutes, until the marshmallows have puffed up and melted together just a little.

While marshmallow-topped cake is in the oven, stir together the powdered sugar, cocoa powder, evaporated milk, and butter, and salt until very smooth. Remove cake from the oven and pour the topping over the hot cake.

Let the cake cool for at least 1 hour (cooling it fully to room temperature makes it much easier to cut), then slice and serve.