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1
Preheat oven to 350u00b0F. Pierce sweet potato with fork; roast until tender, 1 to 1 1/2 hours, depending on width of potato. Cool. Cut potato in half and scoop flesh into mini processor; puree until smooth. Measure 1 cup puree (reserve any remaining puree for another use). Maintain oven temperature.
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2
Stir 1/2 cup sugar and 2 tablespoons water in medium saucepan over medium-low heat until sugar dissolves. Increase heat to high and boil without stirring until syrup turns deep amber, occasionally swirling pan and brushing down sides with wet pastry brush, about 5 minutes. Immediately pour caramel into 8-inch-diameter metal cake pan, leaving about 2 tablespoons caramel in saucepan. Using pot holders, swirl cake pan, allowing caramel to coat bottom and about 1/2 inch up sides.
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3
Add half and half to remaining caramel in saucepan. Stir over medium-low heat until caramel dissolves. Whisk eggs in medium bowl until frothy. Whisk in cream, salt, 1 cup sweet potato puree, and remaining 1/2 cup sugar. Gradually whisk in hot half and half mixture. Strain into same saucepan. Stir over medium heat 1 minute. Remove from heat; whisk in Port. Pour custard into prepared cake pan.
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4
Place cake pan in large roasting pan. Add enough hot water to roasting pan to come halfway up sides of cake pan. Bake until just set in center, about 1 hour. Remove flan from water. Chill until cold, about 5 hours.
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5
Dip bottom and 1 inch up sides of cake pan into pan of hot water 15 seconds. Wipe dry. Invert onto rimmed plate, scraping caramel in pan over flan.
