To prepare a sweet potato layer cake with spiced pecan buttercream recipe, first you need to prepare the sweet potatoes. You will need: Sweet potatoes, sugar, butter, eggs, vanilla extract, flour, baking powder, salt, cinnamon, and nutmeg. Preheat the oven to 350?F (180?C) and grease and flour two 8-inch round cake pans. In the bowl of an electric mixer, cream together the butter and sugar until light and fluffy. Add the eggs and vanilla and beat until combined. Gradually add the flour mixture and mix until the mixture is just combined and no streaks of flour remain. Pour the batter into the prepared pan and bake the cake for 35 to 40 minutes, or until toothpick inserted in the center comes out clean. Allow the cakes to cool for 10 minutes before removing them from the pan.