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Place rack in center of oven and preheat oven to 350 degrees.
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Microwave water for one minute in a bowl. Add 3 oz chopped unsweetened chocolate and stir with a fork or whisk until smooth. Set aside.
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Place room temperature butter in a mixing bowl or in the bowl of a stand mixer. Turn the mixer on and cream butter and sugar until smooth.
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Add eggs and beat for one minute. Add buttermilk and vanilla mix until smooth.
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Add the melted chocolate mixture to the wet ingredients.
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Add flour, salt, baking soda all at once to bowl. Pulse the mixer until the ingredients are incorporated. When the dry ingredients have disappeared, turn the mixer to medium and mix for one minute.
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Grease a 9x13 pan with cooking spray. Scrape the batter into the pan and bake for 35-40 minutes*