SPICED SWEET_POTATO BUNDT CAKE WITH BROWN SUGAR ICING

4   eight-ounce yams or sweet potatoes (will need 2 cups mashed)
2 3/4 Cups all purpose flour
2 teaspoons ground cinnamon
1 1/4 teaspoons ground ginger
1 teaspoon baking powder
1 teaspoon baking soda
1/2 teaspoon salt
2 cups sugar
1 cup vegetable oil
4 large eggs
1 teaspoon vanilla extract

ICING
1 cup powdered sugar
3/4 cup packed dark brown sugar
1/2 cup whipping cream
1/4 cup (1/2 stick) unsalted butter
1/4 teaspoon vanilla extract

FOR CAKE:
Pierce yams with fork.  Microwave on high until very tender, about 8 minutes per side.  Cool, peel, and mash yams.

Position oven rack in middle of oven.  Preheat oven to 325 degrees F.
Spray a bundt pan with nonstick spray and then generously butter the pan. 
Sift flour, cinnamon, ginger, baking powder, baking soda, and salt into medium bowl.  Measure 2 cups mashed potatoes and transfer to large bowl, reserving any remaining yams for another use.  Add sugar and oil to yams and using an electric mixer, beat until smooth.
Add eggs, 2 at a time, beating well after each addition. 
Add flour mixture and beat just until blended.  Beat in vanilla.

Transfer batter to prepared pan.  Bake cake until tester comes out clean about 1 hour and 5 minutes.  Cook cake in pan on rack 15 minutes.  Run knife around sides of pan and center tube to loosen cake.  Turn cake out onto rack and cool completely.

Icing:
Sift powdered sugar into medium bowl.  Stir brown sugar, whipping cream, and butter in medium saucepan over medium-low heat until butter melts and sugar dissolves.  Increase heat to medium high and bring to boil.  Boil 3 minutes, occasionally stirring and swirling pan.  Remove from heat and stir in vanilla.  Pour brown sugar mixture over powdered sugar.  Whisk icing until smooth and lightened in colour, about 1 minute.  Cool until icing is lukewarm and falls in heavy ribbon rom spoon. whisking often, about 15 minutes.  Spoon icing thickly over top of cake, allowing icing to drip down sides.  Let stand until icing is firm, at least 1 hour.
Can be prepared 1 day ahead.  Cover and let stand at room temperature.