Step 1

Preheat the oven to 350°F and lightly spray a 9x13-inch baking pan with nonstick cooking spray. In a large bowl, whisk together the flour, baking soda, and cinnamon. Set aside.

Step 2

Use an electic mixer to beat together the sugar and oil until well combined. Add the eggs, one at a time, beating well after each addition. Add the flour mixture, all at once, and mix on low speed until well just combined. Scrape down the sides of the bowl, and mix again to ensure everything is combined. Fold in the sweet potatoes and pecans - reserving a few pecans to sprinkle on the top at the end.

Step 3

Pour the batter into the prepared pan. Bake for 50 to 60 minutes, or until a toothpick inserted in the center of the cake comes out clean. Cool completely.

Step 4

Use an electric mixer to beat the butter until smooth. Add the marshmallow cream and mix until combined. Add the powdered sugar and a pinch of salt and mix until combined. Spread the buttercream on the cooled cake. Store the cake in the refrigerator.