Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan or line it with parchment paper.

In a mixing bowl, combine the mashed sweet potatoes, granulated sugar, brown sugar, and vegetable oil. Mix well until smooth and creamy.

Add in the eggs one at a time, followed by the vanilla extract. Mix until everything is well-blended.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, nutmeg, and salt. Gradually add this dry mixture to the wet mixture, stirring until just combined. If using, fold in the chopped pecans or walnuts.

Pour half of the batter into the prepared cake pan and evenly spread it. Sprinkle half of the cinnamon sugar mixture on top.

Add the remaining batter on top of the cinnamon sugar, spreading it gently. Finally, sprinkle the rest of the cinnamon sugar mixture on top.

Use a toothpick or knife to swirl through the cake batter gently, creating a marbled effect.

Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.

Let it cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

Slice and serve.