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Combine the graham cracker crumbs, the butter, and 1/4 cup of the sugar in a small bowl. Pat firmly onto the bottom of a 9-inch springform pan.
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Beat the cream cheese until smooth. {I do this in my stand mixer with the paddle attachment.} Add the remaining 1 & 3/4 cup sugar and mix well.
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Add the eggs, one at a time, beating after each addition.Sir in the sour cream and orange extract. Set aside.
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Place the chocolate chips in a medium, microwave-safe bowl and microwave on high for 1 minute. Stir and continue to microwave in 15 second increments, stirring in between, until chocolate is completely melted.
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Stir 3 cups of the cream cheese mixture into the melted chocolate chips and pour over the crust. Freeze 10-15 minutes, to let the chocolate set up.
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Preheat oven to 325°F.
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Stir the orange peel into the remaining cream cheese mixture and gently spread over the chocolate mixture.
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Bake for 1 hour and 15 minutes or until set except for a 3-inch circle in the center. Turn off the oven and let the cheesecake stand in the oven with the door ajar for one hour. Remove from the oven.
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With a knife, loosen the cheesecake from the side of the pan. Let it cool completely and then remove the side of the pan.
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Cover and refrigerate for at least 4 hours.
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Garnish with whipped topping, if desired.