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1
Preheat the oven to 350 degrees.
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2
Lightly grease and flour a 9x13-inch pan.
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3
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.
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4
Off heat, add the sugar, eggs, vanilla, flour, nuts, and baking powder and mix well.
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5
Press into the prepared pan and set aside.
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6
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.
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7
Scrape down the sides of the bowl, add the nuts, mix to combine, and spread over the first layer.
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8
Sprinkle with the chocolate chips and transfer to the oven.
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9
Bake until slightly golden on the edges, about 20 to 25 minutes.
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10
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.
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11
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.
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12
Add the cream cheese and milk and mix until smooth.
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13
Add the confectioners' sugar and vanilla and mix until smooth.
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14
Immediately pour the frosting over the marshmallows and briefly swirl the two mixtures together.
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15
Cool in the pan and cut into 36 to 48 bars.
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16
Cover and refrigerate.
