Set the oven to 350°F (175°C) so it has time to fully heat. Grease a 9×13-inch baking pan with butter or cooking spray. A sheet of parchment paper also works well if you want the cake to lift out easily after baking.

Add the flour, sugar, cocoa powder, baking powder, and salt to a large mixing bowl and whisk until combined. Crack in the eggs, then pour in the milk, vegetable oil, and vanilla extract. Stir until the batter looks smooth and free of lumps. Slowly pour in the hot coffee or hot water, mixing gently. Finish by folding the chopped pecans or walnuts throughout the batter.

Transfer the batter to the prepared pan and smooth the top into an even layer. Place the pan in the oven and bake for 30 to 35 minutes. Check the center using a toothpick. If it comes out clean or has only a few moist crumbs, the cake is ready. Set the pan on a wire rack.

While the cake is still warm from the oven, sprinkle the miniature marshmallows across the top. Leave the cake alone for about 5 minutes so the marshmallows become soft and slightly gooey from the heat.

In a medium bowl, stir together the melted butter, powdered sugar, cocoa powder, salt, and milk until the frosting becomes rich and smooth. If it feels too thick, stir in a little extra milk, one teaspoon at a time, until it spreads easily.

Spread the chocolate frosting over the softened marshmallows, covering every corner of the cake. Leave it to rest for about 30 minutes so the frosting can firm up slightly. Slice into 12 pieces and serve at room temperature for the richest chocolate flavor and the softest texture.