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Preheat the oven to 400 degrees F. Line baking sheets with parchment or non-stick liner.
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Melt the unsweetend chocolate and set aside to cool.
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Whisk together the flour, baking soda, and salt in a bowl.
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Cream the sugar and butter in a large bowl using a hand mixer on high speed.
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Add the buttermilk, vanilla, egg, and cooled chocolate to the sugar/butter mixture. Beat to combine.
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Add the flour and beat until just combined. Do not over-mix.
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Stir in the chocolate chips.
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Drop 2 teaspoons of dough two inches apart on prepared baking sheets. Press down dough to about 1/4-inch thick and shape into rounds.
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Bake 8 to 10 minutes, until almost no indention remains when touched.
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Immediately transfer cookies to a cooling rack.
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Repeat with remainig dough. Make sure baking sheets are completely cool between batches.
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Cool cookies completely before assembling into sandwich cookies with filling.