Preheat the oven to 350 degrees. Lightly grease and flour a 9x13-inch pan.
To make the first layer: Place the butter and chocolate in a small pan and cook over the lowest possible heat until almost all the chocolate has melted. Off heat, add the sugar, eggs, vanilla, flour, nuts, and baking powder and mix well. Press into the prepared pan and set aside.
To make the second layer: Place the cream cheese, sugar, flour, butter, egg, and vanilla in the bowl of a mixer fitter with a paddle and mix until smooth and fluffy. Scrape down the sides of the bowl, add the nuts, mix to combine, and spread over the first layer. Sprinkle with the chocolate chips and transfer to the oven. Bake until slightly golden on the edges, about 20 to 25 minutes.
Sprinkle the second layer with the marshmallows and return the pan to the oven until the marshmallows are just melted but not colored, about 3 minutes.
To make the frosting: Place the butter and chocolate in a small pan and cook over the lowest possible heat until almost all the chocolate has melted. Add the cream cheese and milk and mix until smooth. Add the confectioners' sugar and vanilla and mix until smooth. Immediately pour the frosting over the marshmallows and briefly swirl the two mixtures together. Cool in the pan and cut into 36 to 48 bars. Cover and refrigerate.