Bake the sweet potatoes until tender. Allow them to cool enough to handle, remove the skins, and mash until smooth. Measure 1 cup for the recipe.
Preheat the oven to 350°F. Prepare a 9×13-inch baking dish with nonstick spray.
In a large bowl, mix together the mashed sweet potato, eggs, sour cream, and vegetable oil. Add the yellow cake mix and stir until combined.
In a separate bowl, combine the brown sugar, cinnamon, nutmeg, and chopped pecans.
Spread half of the cake batter into the prepared baking dish. Sprinkle the brown sugar mixture evenly over the batter, then spread the remaining cake batter over the top.
Run a knife gently through the layers a few times to create a swirl. Do not completely mix the layers together.
Bake for 30–35 minutes, or until the center is cooked through and a toothpick inserted into the cake comes out clean.
While the cake cools briefly, whisk together the powdered sugar, milk, and vanilla until smooth.
Spread the glaze over the cake while it is still warm. The glaze will spread easily and set into a thin icing as the cake cools.
Allow the cake to cool and the icing to set before slicing and serving.