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Preheat oven to 425 degrees F.
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Prepare a 9-inch square baking pan by lining it with foil and spraying the foil with nonstick cooking spray or brushing it with vegetable oil.
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Melt the semisweet chocolate and the butter in a double boiler (a pan set over a hot water bath); stir until smooth.
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Add vanilla and cinnamon; stir thoroughly to combine.
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Remove mixture from the heat and cool to room temperature.
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In the bowl of your stand mixer, beat the eggs on high speed until very frothy and about triple in volume (approximately 5 minutes).
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Fold half of the beaten eggs into the chocolate mixture and combine.
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Then fold in the remaining egg.
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Pour the batter into the prepared baking pan and smooth the top with a spatula.
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Place the pan of brownie batter into a larger pan, like a roasting pan and fill that pan with hot water half way up the side of the smaller pan containing the batter.
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Place the pan on the middle rack of the preheated oven and bake for 15 minutes.
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Turn off the oven but keep the brownies in the closed oven for another 10 minutes.
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After 10 minutes, remove the brownies pan from the larger water bath pan and place it on a wire rack to cool while making the glaze.
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To make the glaze, heat the cream and the dark chocolate in a double boiler over simmering water.
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Stir until the chocolate is melted and the mixture is smooth.
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Pour the hot glaze over the brownies and smooth it out with a spatula.
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Set the pan aside to cool completely.
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When cool, cover the pan with plastic wrap and refrigerate overnight.
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Cut chilled brownie into 12 squares.
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Brownies taste best when allowed to come to room temperature for 30 minutes before serving.