For the bars: Line a 9-by-13-inch baking pan with parchment so that the edges fold over the sides of the pan. Spray with cooking spray.
Add the sweetened condensed milk, cream cheese and butter to a medium pot and place over low heat. Cook, whisking occasionally so the bottom does not brown, until melted and well combined, a few minutes.
Put the pecans, walnuts, graham crackers and marshmallows in a bowl; set aside.
When the cream cheese mixture has melted, whisk until well combined, and then remove from the heat. Fold in the chocolate chips and mix until all the chocolate is melted and the mixture is well combined.
Pour the chocolate mixture into the bowl with the graham cracker mixture. Pour the mixture into the prepared pan and place in the fridge for 1 to 1 1/2 hours to set.
For the quick marshmallow frosting: In the bowl of stand mixer fitted with the paddle attachment, beat the sugar and butter until creamy, a couple of minutes. Beat in the marshmallow creme.
Remove the set mixture from the pan. Cut into squares and frost with or dip in the marshmallow frosting. Keep refrigerated.