Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan, or line it with parchment paper for easy removal.

In a large saucepan, melt 1 cup of butter over low heat. Stir in granulated sugar and 1/3 cup cocoa powder until well combined. Remove from heat and let cool slightly.

Beat in eggs one at a time, then stir in vanilla extract. Gradually add flour and salt to the chocolate mixture, stirring until just combined. Fold in nuts if using.

Pour the batter into the prepared pan and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

As soon as the cake is done, remove it from the oven and cover the top evenly with mini marshmallows. Return the cake to the oven for an additional 2-3 minutes, or until the marshmallows are slightly melted. Remove from the oven and let cool.

For the frosting, combine 1/2 cup butter, milk, and 1/3 cup cocoa powder in a saucepan. Heat over low heat, stirring constantly, until the butter is melted. Remove from heat and stir in vanilla extract.

Gradually whisk in powdered sugar until smooth. If the frosting is too thick, add a little more milk until you reach the desired consistency.

Pour the warm frosting over the cooled cake and marshmallow layer, spreading evenly with a spatula. Allow the frosting to set before slicing.

Cut the cake into squares and serve. The cake is rich, so consider serving smaller portions.

Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan, or line it with parchment paper for easy removal.

In a large saucepan, melt 1 cup of butter over low heat. Stir in granulated sugar and 1/3 cup cocoa powder until well combined. Remove from heat and let cool slightly.

Beat in eggs one at a time, then stir in vanilla extract. Gradually add flour and salt to the chocolate mixture, stirring until just combined. Fold in nuts if using.

Pour the batter into the prepared pan and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

As soon as the cake is done, remove it from the oven and cover the top evenly with mini marshmallows. Return the cake to the oven for an additional 2-3 minutes, or until the marshmallows are slightly melted. Remove from the oven and let cool.

For the frosting, combine 1/2 cup butter, milk, and 1/3 cup cocoa powder in a saucepan. Heat over low heat, stirring constantly, until the butter is melted. Remove from heat and stir in vanilla extract.

Gradually whisk in powdered sugar until smooth. If the frosting is too thick, add a little more milk until you reach the desired consistency.

Pour the warm frosting over the cooled cake and marshmallow layer, spreading evenly with a spatula. Allow the frosting to set before slicing.

Cut the cake into squares and serve. The cake is rich, so consider serving smaller portions.