Preheat oven to 325°F.
Coat a 6-cup nonstick Bundt pan with cooking spray and dust with 1 tbsp flour. Set aside.
In a small bowl, stir together sweet potato puree and buttermilk; set aside.
In a large bowl, combine the brown sugar, corn syrup and oil. Beat with a mixer at medium speed for 5 minutes. Add the egg and beat well.
In another large bowl, sift together the remaining flour, cocoa, baking powder, baking soda and salt. Set aside.
With the mixer at low speed, alternate adding 1/3 of the flour mixture and 1/3 of the sweet potato mixture into the sugar and egg mixture until all ingredients are incorporated (do not overmix). Pour the batter into the prepared pan. Bake in the center oven rack for 40 minutes or until the cake springs back when lightly touched in the center.
Cool at least 15 minutes before removing from pan. Allow the cake to cool on a cake rack for 5 more minutes. Sift the confectioners' sugar over the top of the cake and serve warm. Garnish with fresh raspberries and a sprig of mint.