INGREDIENTS
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1 stick (1/2 cup) unsalted butter
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12 oz semisweet chocolate bars or semisweet chocolate chips
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1 1/2 cups flour
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1/4 cup unsweetened cocoa powder
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1 1/2 teaspoons baking powder
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1/4 teaspoon salt
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1 1/4 cups (packed) light brown sugar
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3 eggs, at room temperature
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1 1/2 teaspoons pure vanilla extract
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30 marshmallows
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7.5 ounce semisweet chocolate bars cut into 1/2 inch squares for garnish