Preheat Oven:

Preheat to 350°F (175°C).

Grease and flour a 9x13-inch baking dish.

Mix the Cake Batter:

In a large bowl, beat together cake mix, mashed sweet potato, sour cream, oil, eggs, vanilla, cinnamon, and nutmeg until smooth and well combined.

Make the Swirl Mixture:

In a small bowl, mix brown sugar and cinnamon.

Assemble the Cake:

Pour half of the batter into the prepared pan and smooth it out.

Sprinkle the cinnamon-sugar swirl evenly over the batter.

Top with the remaining cake batter and gently swirl with a knife if desired (or leave layered for a distinct ribbon).

Bake:

Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean.

Let cool slightly before glazing.

Make the Glaze:

In a small bowl, whisk together powdered sugar, milk, and vanilla until smooth.

Drizzle over the warm cake and let it set.