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Preheat oven 350.
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Chop bittersweet and unsweetened chocolates.
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Place chocolate and butter in large heatproof bowl over slightly smaller saucepan of hot (not boiling) water. Melt, stirring until about three-quarters melted.
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Remove from heat and let finish melting; stir until smooth.
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Whisk in sugar, then eggs, 1 at a time, and vanilla.
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In separate bowl, whisk together flour, baking powder and salt.
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Pour over chocolate mixture; stir just until combined.
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Sprinkle chocolate chips, and marshmallows (optional) and walnuts (optional) over batter; stir to combine.
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Drop by heaping tablespoonful about 2 inches apart onto parchment paper-lined or ungreased rimless baking sheets.
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Bake, 1 sheet at a time, in centre of oven for 10 to 12 minutes or until firm around edges but still soft in centre.
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Let firm up on pan on rack for 5 minutes; transfer cookies to rack to finish cooling.