Cook sweetpotato in boiling water until tender.  Cool, peel and mash until you have 1 cup puree.
In a medium saucepan combine evaporated and regular milk, 1 cup sugar and cinnamon stick.  Heat to just boiling.  Remove from heat and discard cinnamon.  Set aside to cool.

Preheat oven to 350°.  Position oven rack at lowest position.

To make a caramel sauce for flan pan:  heat remaining 1/2 cup sugar over medium high heat in a heavy skillet or saucepan.  Cook, shaking pan and stirring until sugar turns a deep golden brown.  Immediately pour into 6-cup ring mold and, using a pot holder to hold pan (the caramelized sugar is hot), tilt to coat bottom and about halfway up the side with caramel.

The flan cooks in a water bath; boil 3-4 cups water in another saucepan for water bath and reserve.  Meanwhile, with an electric mixer, beat sweetpotato puree and eggs, combining well. Carefully pour cooled milk/sugar combination into sweetpotato puree in a slow stream while continuing to mix.  Pour into prepared ring mold.  Place mold in another, larger pan and set double pans on lowest rack.  Carefully pour boiling water into larger pan to come 1/2 to 2/3 up the sides of the flan pan. 

Bake 40-45 minutes until custard is set.  Cool on a rack.  Cover and refrigerate for several hours or overnight.  When ready to serve, run a small knife along the edge of pan.  Invert on a plate. 

Serve with Pears Flambé, recipe below:

Pears Flambé:
6 pears
2 tablespoons lemon juice
4 tablespoons butter
1/4 cup sugar
1/4 cup brandy

Peel, core, and slice pears lengthwise; place in a bowl and toss gently with lemon juice to prevent them turning brown. 
Melt butter in a large skillet and add sugar.  Cook over high heat to dissolve and begin caramelizing sugar.  When sugar just starts to turn brown, stir in pears and continue to cook, stirring gently for 3 minutes.  Pour in brandy and carefully ignite with a match.  Continue to cook until brandy cooks off and flame goes out.  Spoon pears and sauce into the center of flan, and serve.