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1
Place rack in the middle of the oven; heat to 350 degrees. Spray a 12-cup Bundt pan with vegetable oil and set aside.
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2
Sift flour into a bowl. Add brown sugar, potato starch, salt, cinnamon and baking soda. Whisk to blend; set aside.
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3
Put the sour and heavy creams, 1/4 cup dark rum, the molasses and vanilla in a separate bowl. Whisk to blend; set aside.
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4
Beat the butter and sugar on the lowest speed of an electric mixer for 2 to 3 minutes.
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5
With the mixer still on low, add the vanilla, eggs and yolks, one at a time, fully incorporating after each addition. Stop the mixer and scrape down the sides of the bowl.
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6
Add the dry ingredients alternately with the cream mixture in 3 additions each, beginning and ending with the flour mixture. Don't wait for them to be fully incorporated between additions. This step should take about 60 seconds.
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7
Stop the mixer and scrape down the sides of the bowl. Mix on medium speed for 15 to 20 seconds.
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8
Fill the prepared pan by depositing the batter in small clumps around it with a rubber spatula. Level the batter with the spatula.
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9
Bake 50 minutes. Test for doneness by inserting a bamboo skewer into the center. When the skewer comes out clean, the cake is done. Remove pan from oven and place on a wire rack. When the cake has cooled 10 minutes, invert it onto the rack and remove the pan. Let it cool to room temperature.
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10
To glaze the cake, set the wire rack over wax paper. Combine simple syrup with remaining 1/4 cup rum and use a pastry brush to brush it all over the cake until it is absorbed.
