For 1st layer:
Start with bottom layer. Preheat oven to 350°F. Lightly grease a 9 x 13 pan. Melt butter and chocolate over low heat. Sift together the dry ingredients: flour and baking powder. Once chocolate is melted, add sugar, eggs, vanilla, dry ingredients and pecans. Mix until well combined. Press into the prepared pan and set aside.
Second Layer: Cream together the cream cheese and sugar. Add flour, butter, eggs and vanilla. Beat until smooth. Scrape down bowl and add the pecans and mix to combine. Spread the cream cheese layer over the bottom chocolate layer. Sprinkle with the chocolate chips. Bake in oven until only slightly golden on the edges, approximately 20-25 minutes.
Third Layer: Sprinkle with marshmallows and return the pan to the oven until the marshmallows are just melted without color, approximately 3 minutes.
Frosting: 1. Melt the butter and chocolate over low heat. Cream the cream cheese. Add the melted chocolate mixture to the cream cheese. Add the milk. Add the confectioner’s sugar and vanilla and mix until smooth. Pour oven the hot marshmallows and swirl the frosting with the melted marshmallows. Sprinkle more nuts on top if desired. Cool in the pan.