Preheat your oven to 350°F (175°C).
Prepare a 9x13 inch baking dish by greasing it lightly with butter or cooking spray.
In a medium bowl, combine the mashed sweet potatoes with the sugars and softened butter. Mix until well blended.
Add eggs one at a time, beating well after each addition. Mix until smooth.
In another bowl, whisk together the flour, baking powder, cinnamon, nutmeg, and salt.
Gradually add the dry mixture to the wet ingredients, mixing just until combined.
Pour half of the batter into the prepared baking dish and spread evenly.
In a small bowl, mix remaining sugar with cinnamon to create a filling. Sprinkle it over the first layer of batter.
Top with the remaining batter and gently spread to cover the cinnamon-sugar filling.
Use a knife to swirl through the batter lightly for a marbled effect.
Bake in the preheated oven for 30-35 minutes or until a toothpick inserted into the center comes out clean.
Once baked, remove from the oven and allow it to cool in the pan for 10 minutes before transferring to a wire rack.
Prepare the glaze by mixing powdered sugar and milk in a bowl until smooth.
Drizzle the glaze over the cooled cake once it’s completely cooled.