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In a mixing bowl, cream butter and sugar.
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Add the eggs, one at a time, beating well after each addition.
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Beat in the chocolate syrup and vanilla.
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Add the flour and salt until blended.
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Pour into a greased 15x10x1-inch baking pan.
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Bake at 350 degrees for 20-25 minutes or until a toothpick inserted near the center comes out clean (top of brownies will still appear wet).
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Cool on a wire rack for 15-20 minutes.
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Meanwhile, in a small saucepan, combine 1 cup sugar, 1/3 cup butter, and 1/3 cup milk.
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Bring to a boil; boil until the sugar is dissolved.
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Remove from heat and stir in chocolate chips and marshmallows until melted.
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Pour over the brownies and quickly spread evenly.
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Refrigerate for 5 minutes before cutting.